Silicon carbide power devices

ABSTRACT

An embodiment relates to a device comprising a unit cell on a SiC substrate, the unit cell comprising a gate insulator film, a trench in the well region, and a first sinker region of a second conduction type, wherein the first sinker region has a depth that is equal to or greater than a depth of a well region; wherein the device has an on-resistance of less than 3 milliohm-cm 2 , a gate threshold voltage of greater than 2.8V, a breakdown voltage of greater than 1450V, and an electric field of less than 3.5 megavolt/cm in the gate insulator film at a drain voltage of less than or equal to 1200 V.

FIELD OF THE INVENTION

This invention relates to power semiconductor devices using a verticalsilicon carbide (SiC) Double-Implantation Metal oxide semiconductorfield-effect transistor (DMOSFET). A power metal oxide semiconductorfield-effect transistor (MOSFET) is a specific type of MOSFET designedto handle significant power levels.

BACKGROUND OF INVENTION

Silicon based power devices have long dominated power electronics andpower system applications. On the other hand, SiC is a wider band-gap(Eg) material with Eg=3.3 eV as compared to silicon (Eg=1.1 eV);therefore, SiC has a higher blocking voltage than Si. SiC has a higherbreakdown electric field (3×106 V/cm to 5×106 V/cm) compared to Si(breakdown electric field for Si is 0.3×106 V/cm) and is a betterthermal conductor (3.7 (W/cm-K) for SiC versus 1.6 (W/cm-K) for Si). SiChas been a material of choice for power MOSFETs. However, “[e]ven withthe successful introduction of SiC power MOSFETs into the commercialmarket place, several key reliability issues have not been fullyresolved.” [Source: Key Reliability Issues for SiC Power MOSFETs, A.Lelis, D. Habersat, R. Green, and E. Mooro of the U.S. Army ResearchLaboratory, published in ECS Transactions, 58 (4) 87-93 (2013), DOI:10.1149/05804.0087ecst].

SiC exists in a kind of polymorphic crystalline building known as apolytype, e.g. 3C-SiC, 4H-SiC, 6H-SiC. FIG. A1 a is the prior art SiCDMOSFET structure reported by B. J. Baliga in Advanced High-VoltagePower Device Concepts (Springer Press, 2011). FIG. A1 b shows theelectric field contours simulated for the prior art SiC DMOSFETstructure of FIG. A1 a. The electric field distribution near the surfaceof the 5-kV shielded 4H-SiC inversion-mode power MOSFET structure isshown in FIG. A1 b to allow examination of the electric field in thejunction gate field-effect transistor (JFET) region and the gate region.The sharp peak of the electric field at the edge of the P+ shieldingregion can be observed in this prior art device which will result in ahigh electric field in the gate oxide, thereby resulting in a poorperformance of this MOSFET device. FIG. A1 c shows the electric fielddistribution in the shielded 4H-SiC inversion-mode MOSFET. The simulatedresults in the figure show an electric field as high as 4 MV/cm in thegate oxide for the prior art SiC DMOSFET structure of FIG. A1 a.

A typical SiC MOSFET device structure such as that shown in FIG. A1 aresults in high electric field concentration at the corner of the p-wellregion, which results in a high electric field in the gate oxide layer,especially during high drain bias (blocking mode) operation. The highcritical electric fields for breakdown in 4H-SiC (≈3 MV/cm) results in avery high (>5 MV/cm) electric field in the gate oxide. Fowler-Nordheimtunneling currents are observed at such high electric fields in the gateoxide, which can result in trapped charge in the gate oxide, which leadsto poor device reliability.

The manufacturing processes for Si uses techniques like diffusion ofdopants but these conventional manufacturing processes are not possiblefor making SiC device because the diffusion coefficients in SiC arenegligible at temperatures below 1800° C. SiC devices are manufacturedby ion implantation of both source and p-well regions but ionimplantation and deep ion implantation is difficult in SiC. Therefore,there is a long-felt need for improved power devices that address thereliability issues for SiC power MOSFETs.

Due to the limited (10-25 cm2/Vs) MOS channel mobility achievable on thestate-of-the-art SiC planar DMOSFETs, it is necessary to form MOSchannels with sub-micron channel lengths so that the overall ONresistance of the power MOSFET is not significantly degraded. “[I]f thep-well regions and N+ source regions were formed using different masks,the misalignment of two masks would result in a different channel lengthon each side of the cell. To avoid the decrease of threshold voltage(Vth) and degrade the breakdown voltage (Vbr), the N+ mask is in aself-aligned fashion with respect to the P-well.” [source: Design andFabrication of 1.2 kV 4H-SiC DMOSFET by R. Huang et al. published in2016 13th China International Forum on Solid State Lighting:International Forum on Wide Bandgap Semiconductors China (SSLChina:IFWS)]. The MOSFET channel region is formed as a result of the offsetbetween the p-well and the N+ source regions in a DMOSFET structure. Ifthe p-well and N+ source regions are formed by two separate maskingsteps, there will inevitably be a certain amount of lithographicmisalignment between these levels, resulting in different (orasymmetric) MOS channel lengths on the two sides of the unit cell. Thelithographic misalignment between two masking levels using projectionlithography techniques typically used in high-volume semiconductormanufacturing can range from +/−0.05 μm to +/−0.2 μm or greater, whichsets a lower limit on the practically realizable MOS channel lengthswithout significant asymmetry. For a target channel length of 0.5 μm, a+/−0.2 μm mis-alignment between the N+ source and p-well masking stepscan result in a MOS channel length of 0.3 μm on one side of the unitcell and a MOS channel length of 0.7 μm on the other side of the unitcell. While the ON resistance of the MOSFET is increased at higher MOSchannel lengths, lower than optimal MOS channel lengths can result inundesirable effects such as gate threshold voltage (Vth) degradation andother short-channel effects such as drain-induced barrier lowering(DIBL).

Self-aligned techniques for eliminating the misalignment between p-welland N+ source regions have been proposed in the literature. Self-alignedMOS channel formation with channel length defined by sidewall spacerdeposition and etching is one such technique. FIG. B1 is the prior artprocess flow of self-aligned implantation technique with channel lengthdefined by sidewall spacer deposition and etching as reported by R.Huang et al. in “Design and Fabrication of 1.2 kV 4H-SiC DMOSFET”.

Additionally, the “[t]hreshold voltage of the power MOSFET is animportant design parameter from an application stand-point. A minimumthreshold voltage must be maintained at above 1 volt for most systemapplications to provide immunity against turn-on due to voltage spikesarising from noise. At the same time, a high threshold voltage is notdesirable because the voltage available for creating the charge in thechannel inversion layer is determined by (V_(G)−V_(T)) where V_(G) isthe applied gate bias and V_(T) is the threshold voltage.” [source: B.J. Baliga, Silicon Carbide Power Devices, Springer Press (2005), Page234]

FIG. C1 [source: B. J. Baliga, Silicon Carbide Power Devices, SpringerPress (2005), Page 235] shows the threshold voltage of 4H-SiC planarMOSFETSs for the case of a gate oxide thickness of 0.1 microns. Theresults obtained for a silicon power MOSFET with the same gate oxidethickness is also provided in the figure for comparison.

In the race to achieve lower R_(DS,ON) of planar gate SiC MOSFETs withhigh breakdown voltage ratings, it is a common practice to make thechannel lengths (L_(CH)) as short as possible which reduces a great partof the conduction loss that is associated to the channel. The trade-offin doing so is that, as the channel lengths are becoming shorter, theMOSFET devices become susceptible to undesirable phenomena such as theDIBL effect (the Drain Induced Barrier Lowering effect) which isresponsible for the poor device performance including but not limited tothe roll-off of the threshold voltage (V_(TH)) at high drain bias andthe increase of the drain leakage (I_(L)) at high drain bias. Aconventional approach to mitigate this problem is to uniformly increasethe doping concentration in the channel region, but this approachsuffers from a higher than optimal gate threshold voltage andon-resistance, which can offset the gains achieved from the channellength reduction.

SUMMARY OF INVENTION

An embodiment relates to a device comprising a unit cell on a SiCsubstrate, the unit cell comprising a gate insulator film, and a firstsinker region of a second conduction type, wherein the first sinkerregion has a depth that is equal to or greater than a depth of a wellregion; wherein the device has an on-resistance of less than 3milliohm-cm², a gate threshold voltage of greater than 2.8V, a breakdownvoltage of greater than 1450V, and an electric field of less than 3.5megavolt/cm in the gate insulator film at a drain voltage of less thanor equal to 1200 V.

Other embodiments relate to the following: wherein the device comprisesa semiconductor metal-insulator-semiconductor transistor component;wherein the unit cell further comprises a semiconductor body of a firstconduction type that comprises a drift zone; the well region of thesecond conduction type next to an insulator-semiconductor interface; anda source region of a first conduction type formed within the wellregion; wherein the device further comprises a trench in the wellregion; wherein a depth of the trench is greater than or equal to athickness of the source region; wherein the first sinker region islocated below the trench; wherein the device further comprises a secondsinker region of the second conduction type; wherein a depth of thesecond sinker region is less than a depth of the first sinker region;and wherein a depth of the second sinker region is greater than a depthof the well region.

Another embodiment relates to a device comprising a unit cell on a SiCsubstrate, the unit cell comprising a well region, a source region, anda trench, wherein a depth of the trench is greater than a depth of thesource region; wherein the device has an avalanche energy of greaterthan 15 J/cm², calculated by dividing an avalanche energy in joules by atotal die area in cm².

Other embodiments relate to the following: wherein the unit cell furthercomprises a first sinker region of a second conductivity type, whereinan avalanche failure is located within the unit cell.

Another embodiment relates to a device comprising silicon carbide, thedevice having a second conductivity type shield region that is outside ajunction field-effect transistor region, wherein a doping concentrationin a second conductivity type well region within a MOSFET channel isnon-uniform wherein the device has a gate threshold voltage of greaterthan 3.8 V, a breakdown voltage of greater than 4100 Volt, anon-resistance of less than 12 milliohm-cm², an electric field of lessthan 3.5 megavolt/cm at less than or equal to a drain voltage of 3500Volt, and a short-circuit withstand time of greater than 6 μs at a drainvoltage of 1500 Volt.

Other embodiments relate to the following: wherein the secondconductivity type shield region is located inside the secondconductivity type well region in a manner that a lateral location of apoint having a higher doping concentration than that of an averagebackground doping concentration of the second conductivity type wellregion is positioned within the second conductivity type well region;wherein the second conductivity type shield region extends beyond thesecond conductivity type well region; wherein the second conductivitytype shield region extends beyond a vertical extent of the secondconductivity type well region; wherein the device comprises multiplesecond conductivity type shield regions; wherein doping concentrationprofiles of the second conductivity type shield region in differentregions are different; and wherein doping concentration profiles of thesecond conductivity type shield region in different regions are notsubstantially different.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. A1 a shows the prior art SiC DMOSFET structure reported by B. J.Baliga in Advanced High-Voltage Power Device Concepts, Springer Press,2011.

FIG. A1 b shows the electric field contours simulated for the prior artSiC DMOSFET structure in FIG. A1 a.

FIG. A1 c shows the electric field distribution for the prior art SiCDMOSFET structure in FIG. A1 a.

FIG. A2 a shows an embodiment of a SiC DMOSFET with the P+ plug regionto ground the p-well region with the N+ source contact.

FIG. A2 b shows the breakdown simulation of the SiC DMOSFET structure inFIG. A2 a.

FIG. A3 shows an embodiment of a SiC DMOSFET where the P+ plug region inFIG. A2 a is replaced with a deep P-type Sinker #1 region.

FIG. A4 a to FIG. A4 r are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. A3.

FIG. A5 a shows an embodiment of a SiC DMOSFET where a P-type Sinker #2region under the N+ source region is formed in addition to the deepP-type Sinker #1 region.

FIG. A5 b shows the breakdown simulation of the SiC DMOSFET structuredesigned according to embodiments shown in FIG. A3 and FIG. A5 a.

FIG. A6 a to FIG. A6 j are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. A5 a.

FIG. A7 a shows an embodiment of a SiC DMOSFET where a trench is etchedinto the N+ source region before implanting the P-type Sinker #1 region.

FIG. A1 b shows the breakdown simulation of the SiC MOSFET structure inFIG. A7 a.

FIG. A8 a to FIG. A8 bb are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. A7 a.

FIG. A9 a and FIG. A9 b show the output and breakdown I-Vcharacteristics of 1200 V SiC DMOSFETs fabricated using the teachings ofthis invention.

FIG. A10 a and FIG. A10 b show the transfer (ID v/s VGS) characteristicsof 1200 V SiC MOSFETs fabricated using the teachings of this invention

FIG. A11 is a single-pulse avalanche energy measured for a 1200 V SiCMOSFET fabricated using the teachings of this invention.

FIG. B1 shows the prior art SiC DMOSFET process flow for self-alignedMOS channel formation.

FIG. B2 shows an embodiment of a SiC DMOSFET structure for removal of aparasitic N+ source region formed in the periphery.

FIG. B3 a to FIG. B3 gg are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. B2.

FIG. B4 shows an embodiment of a SiC DMOSFET with a dedicated processstep is utilized for masking the implantation of the N+ source region inthe device periphery.

FIG. B5 a to FIG. B5 gg are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. B4.

FIG. B6 shows an embodiment of a SiC DMOSFET with a dedicated processstep for masking the implantation of the N+ source region in the deviceperiphery as well as the N+ source region in the active region to enableohmic contact to the p-well region

FIG. B7 a to FIG. B7 ff are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. B6.

FIG. B8 shows an embodiment of a SiC DMOSFET where the polysilicon gatemetallization layers are segmented in the peripheral region.

FIG. B9 a to FIG. B9 ff are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. B8.

FIG. B10 is photograph showing a SiC DMOSFET fabricated using theteachings of these inventions and tested for single-pulse avalancheenergy test.

FIG. C1 is the prior art and shows the plot of threshold voltage versusp-base doping concentration for a 4H-SiC planar MOSFET.

FIG. C2 a to FIG. C2 d shows embodiments of a SiC DMOSFET structure forfield shielding within the p-well region.

FIG. C3 a to FIG. C3 u are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. C2 a.

FIG. C4 a to FIG. C4 d show embodiments of a SiC DMOSFET structure forfield shielding formed buried within the p-well structure.

FIG. C5 a to FIG. C5 u are cross sectional views showing the processsteps for manufacturing the SiC DMOSFET structure in FIG. C4 a.

FIG. C6 a and FIG. C6 b are output characteristics of two 3.3 kV SiCMOSFETs fabricated using the teachings of these inventions.

FIG. C6 c is transfer characteristics of two 3.3 kV SiC MOSFETsfabricated using the teachings of these inventions.

FIG. C6 d is a short-circuit test measured for two 3.3 kV SiC MOSFETsfabricated using the teachings of this invention.

DETAILED DESCRIPTION Definitions and General Techniques

Unless otherwise defined herein, scientific and technical terms used inconnection with the present invention shall have the meanings that arecommonly understood by those of ordinary skill in the art. Further,unless otherwise required by context, singular terms shall includepluralities and plural terms shall include the singular. Generally,nomenclatures used in connection with, and techniques of, semiconductorprocessing described herein are those well-known and commonly used inthe art.

The methods and techniques of the present invention are generallyperformed according to conventional methods well known in the art and asdescribed in various general and more specific references that are citedand discussed throughout the present specification unless otherwiseindicated. The nomenclatures used in connection with, and the proceduresand techniques of semiconductor device technology, semiconductorprocessing, and other related fields described herein are thosewell-known and commonly used in the art.

The following terms and phrases, unless otherwise indicated, shall beunderstood to have the following meanings.

The term “unit cell” as used herein refers to a piece of a pattern in asemiconductor which is repeated in the semiconductor.

The term “SiC” as used herein refers to silicon carbide which is acompound semiconductor and is a mixture of silicon and carbon with thechemical formula SiC. Silicon is covalently bonded with carbon. In4H-SiC, 4H is written in the Ramsdell classification scheme where thenumber indicates the layer and the letter indicates the Bravais lattice.That means in a 4H-SiC structure four hexagonal layers of SiC arepresent. SiC exists in a kind of polymorphic crystalline building knownas a polytype, e.g. 3C-SiC, 4H-SiC, 6H-SiC. Presently 4H-SiC is used inpower device manufacturing.

The term “JFET” as used herein refers to junction field-effecttransistor which is a three-terminal semiconductor device that can beused as electronically-controlled switches, amplifiers, orvoltage-controlled resistors. A FET (field-effect transistor) is aunipolar transistor in which current carriers are injected at a sourceterminal and pass to a drain terminal through a channel of semiconductormaterial whose conductivity depends largely on an electric field appliedto the semiconductor from a control electrode. There are two main typesof FETs, a junction FET and an insulated-gate FET. In the junction FET,the gate is isolated from the channel by a pn-junction. In aninsulated-gate FET, the gate is isolated from the channel by aninsulating layer so that the gate and channel form a capacitor with theinsulating layer as the capacitor dielectric.

The term “MOSFET” as used herein refers to metal oxide semiconductorfield-effect transistor. which is a four terminal device with source(S), gate (G), drain (D) and body (B) terminals. The body of the MOSFETis frequently connected to the source terminal so making it a threeterminal device like field effect transistor.

The term “DMOSFET” as used herein refers to double-implantation metaloxide semiconductor field-effect transistor. A common physical structureof SiC MOSFETs is the planar double-implanted MOSFET in 4H-SiC(SiC-DMOSFET).

The term “dopant” as used herein refers to an impurity added from anexternal source to a material by diffusion, coating, or implanting intoa substrate, and changing the properties thereof. In semiconductortechnology, an impurity may be added to a semiconductor to modify itselectrical properties or to a material to produce a semiconductor havingdesired electrical properties. N-type (negative) dopants (e.g., such asphosphorus for a group IV semiconductor) typically come from group V ofthe periodic table. When added to a semiconductor, n-type dopants createa material that contains conduction electrons. P-type (positive) dopants(e.g., such as boron for a group IV semiconductor) typically come fromgroup III and result in conduction holes (i.e., vacancies in theelectron shells).

The term “drain” as used herein refers to the electrode of a fieldeffect transistor which receives charge carriers which pass through thetransistor channel from the source electrode.

The term “source” as used herein refers to the active region/electrodeto which the source of charge carriers is connected in a field effecttransistor.

The term “gate” as used herein refers to the control electrode orcontrol region that exerts an effect on a semiconductor region directlyassociated therewith, such that the conductivity characteristic of thesemiconductor region is altered in a temporary manner, often resultingin an on-off type switching action. The control electrode or controlregion of a field effect transistor is located between the source anddrain electrodes, and regions thereof.

The term “impurity” as used herein refers to a foreign material presentin a semiconductor crystal, such as boron or arsenic in silicon, whichis added to the semiconductor to produce either p-type or n-typesemiconductor material, or to otherwise result in material whoseelectrical characteristics depend on the impurity dopant atoms.

The term “PN junction” as used herein refers to the interface and regionof transition between p-type and n-type semiconductors.

The term “polysilicon” as used herein refers to a polycrystalline formof silicon.

The term “p-type” as used herein refers to extrinsic semiconductor inwhich the hole density exceeds the conduction electron density.

The term “bandgap” as used herein refers to the difference between theenergy levels of electrons bound to their nuclei (valence electrons) andthe energy levels that allow electrons to migrate freely (conductionelectrons). The band gap depends on the particular semiconductorinvolved.

The term “breakdown” as used herein refers to a sudden change from highdynamic electrical resistance to a very low dynamic resistance in areverse biased semiconductor device (e.g., a reverse biased junctionbetween p-type and n-type semiconductor materials) wherein reversecurrent increases rapidly for a small increase in reverse appliedvoltage, and the device behaves as if it had negative electricalresistance.

The term “channel” as used herein refers to a path for conductingcurrent between a source and drain of a field effect transistor.

The term “chip” as used herein refers to a single crystal substrate ofsemiconductor material on which one or more active or passivesolid-state electronic devices are formed. A chip may contain anintegrated circuit. A chip is not normally ready for use until packagedand provided with external connectors.

The term “contact” as used herein refers to the point or part of aconductor which touches another electrical conductor or electricalcomponent to carry electrical current to or from the conductor orelectrical component.

The term “die” as used herein refers to a tiny piece of semiconductormaterial, separated from a semiconductor slice, on which one or moreactive electronic components are formed. It is sometimes called a chip.

The term “sinker” as used herein refers to deep implanted regions at keylocations within the DMOSFET structure

The term “plug” as used herein refers to the structure used to groundthe well and the source contact.

The term “drift layer” as used herein refers to lightly doped region tosupport the high voltage in power MOSFET

The term “well” used herein refers certain regions in ametal-oxide-semiconductor (MOS) transistor. MOS transistors are alwayscreated in a “well” region. A PMOS (positive-channel MOS) transistor ismade in an N-doped region, called “n-well” region. Similarly, an NMOStransistor (negative-channel MOS) is made in a “p-type” region called“p-well”. This ensures that the leakage between two transistors, throughthe bottom side, is low due to the reverse bias between the transistorareas and the well region.

The term “source interconnect metallization” as used herein refers tointerconnection metallization that interconnects many MOSFETs usingfine-line metal patterns.

The term “self-aligned” used herein refers to processing steps inmanufacturing of semiconductor devices. It is often necessary to achieveprecise alignment between structures fabricated at differentlithographic stages of integrated circuit fabrication. Stringentrequirements on lithographic alignment tolerance can be relaxed if thestructures are “self-aligned” which means one is forced into a specificposition relative to the other for a wide range of lithographicallydefined positions.

The term “device” as used herein refers to the physical realization ofan individual electrical element in a physically independent body whichcannot be further divided without destroying its stated function.

The term “surface” as used herein refers to the outer or exteriorboundary of a thing.

The term “trench” as used herein refers to electrical isolation ofelectronic components in a monolithic integrated circuit by the use ofgrooves or other indentations in the surface of the substrate, which mayor may not be filled with electrically insulative (i.e., dielectric)material.

The term “dielectric” as used herein refers to a non-conductor ofelectricity, otherwise known as an insulator.

The term “mobility” as used herein refers to the facility with whichcarriers move through a semiconductor when subjected to an appliedelectric field. Electrons and holes typically have different mobilitiesin the same semiconductor.

The term “RIE” as used herein refers to reactive ion etching which is anetching technology used in microfabrication. RIE is a type of dryetching which has different characteristics than wet etching. RIE useschemically reactive plasma to remove material deposited on wafers. Theplasma is generated under low pressure (vacuum) by an electromagneticfield. High-energy ions from the plasma attack the wafer surface andreact with it.

The term “ILD” as used herein refers to interlayer dielectric adielectric material used to electrically separate closely spacedinterconnect lines arranged in several levels (multilevel metallization)in an advanced integrated circuit.

The term “CVD” as used herein refers to chemical vapor deposition ismethod used to produce high quality, high-performance, solid materials,typically under vacuum. The process is often used in the semiconductorindustry to produce thin films. In typical CVD, the wafer (substrate) isexposed to one or more volatile precursors, which react and/or decomposeon the substrate surface to produce the desired deposit. Frequently,volatile by-products are also produced, which are removed by gas flowthrough the reaction chamber.

The term “PECVD” as used herein refers to plasma-enhanced chemical vapordeposition process used to deposit thin films from a gas state (vapor)to a solid state on a substrate. Chemical reactions are involved in theprocess, which occur after creation of a plasma of the reacting gases.

The term “LPCVD” as used herein refers to low pressure chemical vapordeposition technology that uses heat to initiate a reaction of aprecursor gas on the solid substrate. This reaction at the surface iswhat forms the solid phase material.

The term “DIBL” as used herein refers to drain induced barrier loweringand is a short-channel effect in MOSFETs referring originally to areduction of threshold voltage of the transistor at higher drainvoltages. In a classic planar field-effect transistor with a longchannel, the bottleneck in channel formation occurs far enough from thedrain contact that it is electrostatically shielded from the drain bythe combination of the substrate and gate, and so classically thethreshold voltage was independent of drain voltage. In short-channeldevices this is no longer true: The drain is close enough to gate thechannel, and so a high drain voltage can open the bottleneck and turn onthe transistor prematurely.

The term “p-shield” as used herein refers to a carefully designed p-typedoped region strategically located close to or within the MOSFET channelregion, with the objective of shielding the MOSFET channel from the highpotential applied to the drain terminal during off-state or blockingoperation.

The term “avalanche failure” as used herein refers a phenomenon that canoccur in both insulating and semiconducting materials. It is a form ofelectric current multiplication that can allow very large currentswithin materials which are otherwise good insulators. It is a type ofelectron avalanche. The avalanche process occurs when carriers in thetransition region are accelerated by the electric field to energiessufficient to create mobile or free electron-hole pairs via collisionswith bound electrons. The voltage at which the breakdown occurs iscalled the breakdown voltage. Avalanche failure can cause structuraldamage to a semiconductor device.

The term “avalanche energy” as used herein is defined as the amount ofenergy the MOSFET can withstand when it is set into avalanche mode orits breakdown voltage is exceeded.

The terms “first conductivity type region” and “second conductivity typeregion” as used herein, are used to describe n-type and p-type regionsrespectively for a N type device. For a P type device “firstconductivity type region” and “second conductivity type region” are usedto describe p-type and n-type regions respectively.

Embodiments relate to SiC DMOSFET power devices where the p-well regionseffectively shield the sensitive gate oxide from the high electricfields present in SiC especially during high drain bias or blocking modeoperation.

An embodiment relates to using a P+-plug to ground the p-well regionwith the N+ source contact.

An embodiment relates to making the lateral spacing between the p-wellregions narrow enough to suppress the electric field in the gate oxidewhile ensuring the ON-resistance is not high.

An embodiment relates to replacing the P+ plug region of the DMOSFETwith a deep P-type Sinker #1 region.

Embodiments relate to formation of one or more deep implanted sinkerregions at certain locations within the MOSFET device structure such asa first P-type sinker region at the center of the MOSFET unit cell whosedepth may be equal to or greater than the depth of the p-well region.

Embodiment relates to forming a second P-type sinker region under the N+source region, whose depth may be equal to or greater than the p-wellregion, but less than or equal to the depth of the first P-type sinkerregion.

Embodiment relates to boron implantation which may be advantageouslyused for forming the deep sinker regions since boron has a largerimplant range than aluminum that can result in deeper implant profiles

Embodiment relates to the formation of a first trench with desired shapewhich may be etched in the N+ source region, prior to the formation ofthe first P-type sinker region, which may serve to increase the depth ofthe first P-type sinker region. The depth of the first trench may rangefrom 0.01 μm up to 2 μm. The depth of the resulting first sinker regionmay be 0% to 100% larger than the depth of the p-well region. The depthof the first P-type sinker region can be as large as the entireepitaxial layer.

Embodiment relating to the formation of the first trench in the N+source region may reduce or eliminate the need of expensive ultra-highenergy implantation steps for forming the first P-type sinker region.

Embodiment relating to the first trench may be advantageously used toremove the N+ source implant from the first P-type sinker region, whichmay be desirable to prevent compensation of the first P-type sinkerregion by the N+ source implant. This is especially useful, if the N+source region is self-aligned to the p-well region.

Embodiment relates to a gradually decreasing implant concentration whichmay be employed for forming the first and second P-type sinker regionsin lieu of a box-shaped implant profile, as this may be advantageous inappropriately shaping the electric field under high drain bias. Thedoping in the P-type sinker regions may be varied linearly from amaximum value close to the SiC surface to a value equal to or slightlyhigher than the drift layer doping concentration at the other end of theP-type sinker regions.

Embodiment of a design of the first and second sinker regions maysimplify the design of the p-well region, which can be designed tosupport metal-oxide-semiconductor (MOS) channel formation and may beadvantageously designed for achieving low on-resistance, withoutcompromising other performance metrics, such as reverse leakage currentand electric field in gate oxide.

SiC devices in power electronics feature fast switching times, highblocking voltage capabilities, and the ability to operate at hightemperatures. These characteristics, along with recent advancements inmanufacturing processes, suggest that SiC has the potential torevolutionize power electronics as a successor to traditionalsilicon-based (Si) devices. SiC is a wide band gap material (3.3 eV) andhas higher breakdown electric field (3×10⁶ V/cm to 5×10⁶ V/cm) comparedto Si (Si band gap is 1.1 eV and breakdown electric field for Si is0.3×10⁶ V/cm). SiC is a better thermal conductor (3.7 (W/cm-K) for SiCversus 1.6 (W/cm-K) for Si) which enables SiC devices to operate atextremely high-power levels and still dissipate the large amounts ofexcess heat generated. These material properties of SiC offer multipleadvantages of using SiC instead of Si on power devices. In a comparisonof SiC and Si semiconductor die with identical structures anddimensions, the SiC die exhibit a lower specific ON resistance and ahigher breakdown voltage than the Si die.

The disclosed embodiments herein provide novel techniques for SiCDMOSFET design and fabrication for shaping of the electric field overthe device structure and reducing concentration of electric fields atsingular points. The embodiments herein reduce the electric field in thegate oxide region to less than 3.5 MV/cm and improve the devicereliability.

The manufacturing process in Si uses techniques like diffusion ofdopants but these conventional manufacturing processes are not possiblefor making SiC device because the diffusion coefficients in SiC arenegligible at temperatures below 1800° C. SiC devices are manufacturedby ion implantation of both source and p-well regions but ionimplantation and especially deep ion implantation is difficult in SiC.In the embodiments herein, the ion implantation of source and p-wellregions are made deep with novel techniques.

The increased reliability, higher operating temperature, increasedefficiency, higher voltage capabilities of the SiC devices of theembodiments herein as compared to the silicon devices make them highlydesirable in the electric vehicle and renewable energy industries.Traction inverters in electric vehicles are subjected to high thermal(>150° C.) and load cycling and renewable energy converters aresubjected to extreme environmental conditions. The embodiments describedherein for the SiC devices maximize power conversion efficiency to >98%for example while providing high reliability thus making it an idealcandidate for electric vehicles to minimize maintenance and downtime,which is expensive for the operators of the electric vehicles.

The disclosed embodiments change the way in which SiC power DMOSFETdevices can effectively shield the sensitive gate oxide from the highelectric fields present in 4H-SiC especially during high drain bias(blocking mode operation).

In embodiments herein, SiC power DMOSFET devices overcome the trade-offbetween achieving a low ON resistance and achieving a robust blockingperformance, which implies a low electric field in the structure closeto a gate oxide.

Embodiments herein include a unit cell of a SiC power DMOSFET comprisinga vertical MOSFET. Certain regions of the SiC power DMOSFET device are ap-well region, which is formed by an implantation, an N+ source region,a N− drift layer, and an N+ substrate. In the embodiment herein, duringthe ON state the current flows vertically from the drain, through theinversion layer which is formed at the top of the p-well layer, when agate voltage is applied to this device in through the N+ source regionand out through the source metallization. In the OFF state or theblocking state, in the embodiments herein a voltage is supported acrossthe p-well and N− drift layer junction. There is a PN junction which isformed between the p-well and the N drift layer. The voltage applied tothe structure is supported across this PN junction in the reverse bias.

In the embodiments herein, a power MOSFET has several physicaldimensions, including: the pitch of the unit cell, which is the repeatunit for the MOSFET; the channel length, which is the portion of thep-well in which the inversion channels is formed; the distance betweentwo successive p-wells, referred to as the junction field-effecttransistor (JFET) region or the JFET gap; the thickness of the gateoxide; and an inter-layer dielectric (ILD) layer, which is used toinsulate the source interconnect metallization from the poly-silicongate.

In the embodiments herein, the advantage of using SiC over Si is thatfor similar chip sizes, SiC can support much higher power density. Theamount of voltage one can support in SiC can be ten times more than inSi, and the current that the SiC chip can carry through the MOSFET canbe 10 to 50 times more than in Si. SiC MOSFETs form convenient switch,because one can achieve much higher power density. The performanceadvantage relates to the trade-off between voltage and current. Voltageblocking and current conduction capability, is about 200 times better inthe case of SiC as compared to Si.

In the embodiment herein, the P+ plug, which is in the center of the SiCDMOSFET unit cell, is replaced by the PS #1 region. In the embodimentthe PS #1 region extends below the p-well region. During the blockingcondition, the electric field or the main region where the breakdown canhappen, is at the center of the unit cell of a SiC DMOSFET, which isbasically between the PS #1 region and the N− drift layer. SiCstructures tend to break down between the p-well and the N− drift layer,and especially at the curvature of the p-well region. In the embodimentsherein, the structure will break down between the PS #1 region and theN− drift layer. Here the location of the breakdown has been movedtowards the center of the unit cell which results in a lower electricfield closer to the gate oxide region. There is a reduced electric fieldclose to the gate oxide region in the embodiment due to PS #1 region,and this should make for a more robust device.

The embodiments herein have difference in structure, difference in themethod of manufacturing, difference in the functionality of the devices,and difference in the results that the devices produce than the priorart SiC devices. Simulation data of the embodiments herein show thedifferences in performance and results of structures. In addition to thePS #1 region, the devices of the embodiments herein also have a PS #2region, which is placed under the N+ source region. This improves theelectric field profile in the embodiment in a way that during theblocking mode of operation, the voltage is shed across a larger area ofthe unit cell, which results in even more robust operation. In theembodiments herein, the PS #2 region could be deeper than the p-wellregion and could be made shallower than the PS #1 region. In theembodiment herein of all the P-type regions in the SiC DMOSFETstructure, the deepest of them is the PS #1 region, the intermediate oneis the PS #2 region, and the shallower one is the p-well region. Theembodiments herein show more appropriate electric field shaping fromthese PS #1 and PS #2 structures.

In the embodiments herein, another advantage of including the PS #2region is that the PS #2 serves as a secondary p-well region. The designof the p-well region needs to satisfy several different constraints. Theelectrical dose in the p-well region can be large enough such that thedesired breakdown voltage is realized while at the same time, the dopingof the p-well region near the gate oxide region where the MOS channel isformed, can be low enough such that a reasonably low threshold voltageis realized.

In the embodiment the p-well region could now satisfy conflictingrequirements. By placing the PS #2 under the N+ source region, itrelaxes the design of the p-well region, whose purpose now is solely forchannel definition and channel formation. In embodiments herein, thep-well does not have to be designed with the objective of supporting thebreakdown voltage. The embodiments herein give a lot more flexibility tothe designer to design the p-well region suitably only for MOS channelformation.

In the embodiment herein, the PS #1 is formed after a trench is etchedinto the N+ source region which is in the middle of the unit cell of theSiC DMOSFET. The advantage of this trench is that it moves the electricfield location farther away from the gate oxide interface and to themiddle of the unit cell of the SiC DMOSFET. In the embodiment etching arecess trench into the N+ source region, effectively creates a deeper PS#1 region. The electric field moves even further away from the gateoxide region, which would make for more robust blocking capability. Inthe embodiments herein, one of the reasons for doing the trench isbecause ion implantation and particularly deep ion implantation in SiCis difficult. Deep PS #1 sinker region can be made because of sourcetrench. In the embodiments herein etching a trench and then doing theion implantation, allows to make a deeper PS #1 region, which iseffective and achieves the functionality.

In the embodiments herein since the trench helps in making a deeper PS#1 due to the source trench allows to make a thinner oxide region byreducing the oxide. SiC DMOSFET needs a thicker oxide to prevent themfrom breaking down in a dielectric field. In the embodiments herein byintroducing the P-type sinker regions, one can use thinner oxide.

In the embodiments herein, the SiC DMOSFET devices are made using aseries of masking steps followed by either ion implantation or etchingand a deposition step. In the embodiments herein, the unit cell isproduced where a series of structures are made using photolithographythat is used to mask certain portions of the wafer or certain portion ofdevice, and implementing certain steps, and then removing that mask, anddoing the other steps. To minimize the number of steps is of greatimportance to realize cost-effectiveness. That is, cost structure islower if one can somehow reduce number of steps. Also, certain steps inmanufacturing of SiC are expensive, and the embodiments herein minimizethe number of those expensive steps. For example, ion implantation is arelatively expensive step in SiC. p-well, P-plug, and N+ source regionsare made using ion implantation. Ion implantations are expensive steps,particularly deeper ion implantation is even more expensive. So,minimizing deep ion implantation is of importance. Further, P-typeImplantation is 5 to 10 times more expensive than doing N-typeImplantation, since P-type implantation needs to be performed at highertemperatures (400-1000° C.) in contrast to N-type implants which can beperformed at room-temperature. So, minimizing P-type implants, is alsoof importance.

In the embodiments, herein the wafer goes through a series of ionimplantation steps first, which is typically called the front end of theprocess, then there is a high temperature annealing, which is used toanneal out the implant damage and electrically activate the implants.The back end of the process typically consists of forming the gateoxide, ILD and other metallization steps.

In the embodiments herein to make the SiC DMOSFET structure,photolithography would be used to mask a certain part of the wafer tocreate a p-well region, following which, a next masking step could beused to realize an N+ source region, followed by another masking stepwould be used to realize the P+ plug region. The wafers would need to beannealed to activate the implanted regions. After this, a gate oxide ora gate dielectric can be grown by either thermal oxide or using adeposited oxide. A gate metallization can be formed using a poly-siliconor other refractory metals. The gate metallization is then selectivelypatterned and etched to access the N+ source regions, following whichthe ILD is deposited and then patterned to realize the window forcreating these ohmic contacts for the N+ source regions. The ohmiccontact can be realized on the backside of the wafer, which is thedrain, for the drain region. At the end of the process, a sourceinterconnect metallization is used to connect the various unit cells.

SiC MOSFET is typically produced in several masking steps. Some of thesemasking steps, could be for forming implanted regions, while othermasking steps are for forming structures such as gate oxide, ILD etc.The embodiments herein are all done in 11 masking steps withoutincreasing a masking step for any additional feature like PS #1, PS #2,source trench formations or other structures in the embodiments.

In SiC, ion implantation is a method to introduce dopants sincediffusion does not work well. So, to realize the PS #1 region to bedeeper than p-well region, requires higher implantation energy. In theembodiments herein typically either aluminum or boron is used forrealizing P-type regions in SiC. So, for realizing the PS #1 region anda deep PS #1 region, in the embodiments herein boron implantation isused in lieu of aluminum implantation, since for a given energy, theboron implantation can result in deeper regions as compared to aluminumimplantation. Another way to realize a deep PS #1 region is to use amuch higher energy and still use aluminum as the P-type dope.

In an embodiment herein, a p-well region is formed, following which, theN+ source region is formed. While forming the N+ source region, ionimplantation is performed to realize the PS #2 region. The same maskingstep is used for forming the N+ source region and the PS #2 region. Inthe embodiment herein, this enables one to realize a PS #2 regionexactly under the N+ source region.

In the embodiments herein, the N+ source implant is made self-aligned tothe p-well implant, to reduce or even eliminate misalignment which couldoccur if the two regions are realized in different masking steps.

In the embodiment herein to realize sub 0.5-micron channel lengths orsub 1-micron channel lengths the first p-well region is formed usingtraditional implantation masking step. A dielectric layer of asufficient thickness is deposited, and then using photolithography, onewould selectively pattern this layer, and then perform ion implantation,to realize the p-well region. Once this is done, a second dielectriclayer is deposited on top of this patterned first dielectric layer, andthen subjected to a blanket etch. This, results in a dielectric spacerregion which then defines the region which is subjected to the N+ sourceimplant. So, by choosing an appropriate thickness of the seconddielectric layer, one can define the MOS channel length.

In the embodiment herein to realize a channel which is 0.5-micron widefirst create a p-well region using conventional photolithographytechnique, for example one may deposit and pattern silicon dioxidelayer, then one may deposit a second silicon dioxide layer of athickness equal to like 0.5-micron. When one etches this layer down, onewould realize a spacer which would be 0.5-micron also. When one performsthe N+ source implant, one would have effectively an MOS channel lengthof 0.5-micron.

In the embodiment, the channel length is formed not by the limitationsof lithography techniques, but by the ability of depositing dielectriclayers, for example silicon dioxide, where one has much better control.One can deposit a dielectric or silicon dioxide layers with a resolutionwhich is much better than what lithography can allow.

In one method called the subtractive method, the process firstfabricates a N+ source, then deposits a dielectric layer, and then formthe P+ layer through the poly-silicon variation. The embodiments hereindo the additive method, where first one does the p-well, and thendeposits the dielectric of a controlled thickness, which will define thechannel length, and then put the N+ source. So, the order of N+ andp-well is reversed in the embodiments herein when compared to how priorart SiC DMOSFET's are formed.

The embodiments herein avoid implanting N+ source region in the gateregion. In a SiC MOSFET structure there exists a parasitic NPNtransistor, that can get triggered not during normal operation but whenyou're switching this device from OFF to ON or ON to OFF with extremelyhigh speed.

In the embodiments herein, the trench into the N+ source region isformed prior to the implantation of the PS #1 region results in a deeperPS #1 region. But another benefit of doing this is that one can alsoremove the N+ source regions from the portions of the device that do notwant the N+ source region to be implanted. So, that is the secondarybenefit of having a trench etched into the N+ source region. The samemasking step is used for performing the SiC dry etch, and thenimplanting the PS #1 region. This is achieved with just one maskingstep. In an embodiment, the PS #2 region is formed along with the N+source region with the same masking step. The SiC trench and the PS #1region are again formed using the same masking step. In this embodiment,four features are made using two masking steps.

In the embodiments herein, the cost of PS #1 sinker is reduced, becauseone does not have to employ deep ion implantation. Deep ionimplantation, which are high doses, is expensive. And on the periphery,having a deep P+ is also beneficial, to prevent breakdown regions thatcan be formed. In the embodiments herein, one is avoiding the breakdownregion at the periphery because one is using a single step for creatingPS #1 and the trench. When one forms the MOS channel in a self-alignedfashion, N+ source region is formed everywhere where one has a p-wellregion, which extends to even the periphery of the device. In theembodiments herein, one avoids the breakdown at the periphery due toformation of trench and PS #1.

An embodiment shown in FIG. A2 a is the unit cell of a cross-sectionalstructure of a SiC DMOSFET. The key regions of this device are a p-wellregion A203, which is formed by ion-implantation or epitaxial re-growthof a p-type species such as aluminum or boron. There is an N+ sourceregion A204, N− drift layer A202, and an N+ substrate A201. In the ONstate when a gate voltage is applied to the polysilicon gate A206, thecurrent flows vertically from the drain A201, through the inversionlayer which is formed at the top of the p-well layer A203, through theN+ source region A204, and out through the source metallization A208. Inthe OFF state or the blocking state, a voltage is supported across thep-well A203, N− drift layer A202 junction and there is a PN junctionwhich is formed between the p-well and the N− drift layer. The voltageapplied to the structure is supported across this PN junction in thereverse bias. There are several key features in a power MOSFET, whichinclude the pitch of the unit cell, which is the repeat unit for theMOSFET, the channel length, which is the portion of the p-well in whichthe inversion channel is formed, the distance between two successivep-wells, which is referred to as the JFET region or the JFET gap and thethickness of the gate oxide A205. Another feature is an ILD layer A207which is used to insulate the source interconnect metallization A208from the poly-silicon gate A206. At the very center of the unit cell,there is a P+ plug layer A209 which is grounded with the N+ sourcemetallization. The purpose of the P+ plug in SiC DMOSFET is to groundthe p-well region with the N+ source contact. Typically, the depth ofthe P+ plug implant is made shallower than the p-well region as shown inFIG. A2 a. The DMOSFET device structure can result in high electricfield concentration at the corner of the p-well region A203, whichresults in a high electric field in the gate oxide layer A205,especially during high drain bias (blocking mode) operation. The highcritical electric fields for breakdown in 4H-SiC (≈3 MV/cm) results in avery high (>5 MV/cm) electric field in the gate oxide. Fowler-Nordheimtunneling currents are observed at such high electric fields in the gateoxide, which can result in trapped charge in the gate oxide, which leadsto poor device reliability. In the embodiment herein, the lateralspacing between p-well regions (JFET region) is made narrow enough tosuppress the electric field in the gate oxide while making sure that theMOSFET ON-resistance is low enough.

FIG. A2 b is the breakdown simulation of a SiC MOSFET shown in FIG. A2a. The simulation shows the peak electric field located at the corner ofthe p-well region, which results in a certain high electric field in thegate oxide layer. The electric field is strongly concentrated at thecorner of the p-well region because that is the region of maximumcurvature. (this embodiment has limitation)

An embodiment shown in FIG. A3 is the cross-sectional unit cell of a SiCDMOSFET. The key regions of this device are a p-well region A303, whichis formed by ion-implantation or epitaxial re-growth of a p-type speciessuch as aluminum or boron. There is an N+ source region A304, N− driftlayer A302, and an N+ substrate A301. In the ON state when a gatevoltage is applied to the polysilicon gate A306, the current flowsvertically from the drain A301, through the inversion layer which isformed at the top of the p-well layer A303, through the N+ source regionA304, and out through the source metallization A308. There are severalkey features in a power MOSFET, which include the pitch of the unitcell, which is the repeat unit for the MOSFET, the channel length, whichis the portion of the p-well in which the inversion channel is formed,the distance between two successive p-wells, which is referred to as theJFET region or the JFET gap and the thickness of the gate oxide A305.Another feature is an ILD layer A307 which is used to insulate thesource interconnect metallization A308 from the poly-silicon gate A306.Here the P+ plug region A209 of the SiC DMOSFET from FIG. A2 a isreplaced with a deep P-type Sinker #1 (PS #1) region A309. The depth ofthe PS #1 region, is greater than the depth of the p-well region. Duringthe OFF state or the blocking condition, the structure shown in FIG. 2a, will break down between the p-well A203 and the N− drift layer A202especially at the curvature of the p-well region. But with the PS #1region A309 being introduced in FIG. 3 the electric field or the mainregion where the breakdown will happen, is now at the very center of thedrawing, which is basically between the PS #1 region A309 and the N−drift layer A302. The presence of the deeper PS #1 region results inmoving the peak electric field location from the corner of the p-wellregion A303 to the PS #1 region A309. The location of the peak electricfield in 4H-SiC during high drain bias operation has been moved fartheraway from the gate oxide A305. The electric field in the gate oxide A305is now lower than that of FIG. Ata device.

FIG. A4 a to FIG. A4 r describes the process of manufacturing thestructure shown in FIG. A3. The manufacturing process for a SiC DMOSFETis on a SiC substrate A401 and starts with using a 4H-SiC Si-faceepi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1 μm to300 μm) for the epilayer A402 shown in FIG. A4 a. A blanket hard maskA403 comprising a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm is deposited in FIG. A4 b and thenpatterned using photolithography followed by a dry etch (using areactive ion-etch for example) as shown in FIG. A4 c. A P-typeimplantation A404 comprising boron or aluminum, at energies ranging from10 keV to 800 keV, at implant doses ranging from 10¹² cm⁻² to 10¹⁵ cm⁻²in FIG. A4 d is performed to create a p-well A405 in FIG. A4 e. The maskA403 is removed, and another hard mask layer A406 is depositedcomprising a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm and patterned in FIG. A4 f to define the N+source region. Notice that the center of the unit cell is masked forsource (SRC) implantation. The SRC (N+ source region) A407 is formed byimplantation of the n-type (N+) impurity A408 comprising nitrogen orphosphorus, at energies ranging from 10 keV to 500 keV, at implant dosesranging from 10¹³ cm⁻² to 10¹⁶ cm⁻² as shown in FIG. A4 g. Afterremoving the hard mask A406 another hard mask layer A409 is depositedand patterned in FIG. A4 h. The PS #1 region A411 is formed byimplantation of the p-type impurity A410 in FIG. A4 h. The deep PS #1region can be formed using ion-implantation of aluminum or boron, whichare commonly used p-type dopants in 4H-SiC. Boron can be advantageouslyused for forming this deep P-type Sinker region, since it hassignificantly higher implant range as compared to aluminum, and deepimplants using boron can be formed with lower ion-implantation energies.In one example, the PS #1 region can be formed with boron implantationwith a 20-50% higher depth as compared to the p-well region. The boronimplant may comprise of energies ranging from 10 keV to 800 keV, atimplant doses ranging from 10¹² cm⁻² to 10¹⁵ cm⁻². In a relatedembodiment of this invention, the doping profile of the PS #1 region canbe formed with gradually decreasing doping concentration with a peakvalue at the SiC surface. This graded doping profile for the PS #1region can prevent the formation of sharp corners in the 3-Dimensionallandscape and is beneficial for spreading the peak electric field duringhigh drain bias operation over a larger area which results in a higherbreakdown voltage. In one example, the PS #1 region can be formed with apeak doping in the range of 10¹⁹ cm⁻³ to 10²⁰ cm⁻³ close to the SiCsurface, which linearly decreases as a function of depth into thesilicon carbide to the background N-drift layer doping (in the range of10¹⁴-10¹⁶ cm⁻³). A linearly graded doping profile of the PS #1 regionresults in a sufficient portion of the drain potential being supportedwithin the PS #1 region and not only the N− drift region. This resultsin a pseudo charge-balanced structure, which promotes breakdown over alarger SiC region, which results in a lower critical electric field atbreakdown and consequently a lower electric field in the gate insulator.This is followed by the removal of the hard mask A409 using either dryor wet etching techniques commonly practiced by those in the field inFIG. A4 i. Then the oxide layer A412 which is the gate oxide is eitherthermally grown or deposited using chemical vapor deposition (CVD) inFIG. A4 j. The thickness of the gate oxide could range from 10 nm to 100nm. Either dry or wet thermal oxidation could be used for oxide growth.Plasma-enhanced CVD (PECVD) or low-pressure CVD (LPCVD) could be usedfor gate oxide deposition. A polysilicon gate layer is then deposited onthe top A413 in FIG. A4 k. The polysilicon layer may be deposited usingPECVD or LPCVD. The polysilicon layer may be degenerately doped usingboron or phosphorus, either in-situ or in a subsequent step. In-situdoping may be performed by the addition of PH3 precursor to thepolysilicon deposition chemistry. Post-deposition doping of polysiliconmay be performed by depositing a layer of POCl₃ followed by a drive-instep at temperatures ranging from 700-900° C. A hard mask A414 isdeposited on top and patterned as shown in FIG. A41. The polysilicongate layer A413 is etched by using the patterned mask layer A414 in FIG.A4 m. The mask layer A414 is then removed from the top in FIG. A4 n. Aninterlayer dielectric (ILD) layer A415 comprising 50 nm-1000 nm thicksilicon dioxide, silicon nitride, silicon oxynitride layers or a stackedcombination thereof is deposited on the wafer; a hard mask A416 isdeposited and patterned on top to define the ILD opening; the ILD layeris patterned using the hard mask A416 as shown in FIG. A4 o. Further thegate oxide is etched using the same mask A416 in FIG. A4 p. The maskA416 is then removed and a nickel silicide region A417 is formed on theexposed SiC surface in FIG. A4 q. Interconnect metal layers A418 (eitherAl or Ag or Au) is deposited and patterned on the top and bottom of thechip in. FIG. A4 r

The main problem in the prior art device in FIG. A1 a as recognized bythe authors of this invention is that the p-well regions do noteffectively shield the sensitive gate oxide from the high electricfields present in 4H-SiC especially during high drain bias (blockingmode operation). The presence of the deeper PS #1 region results inmoving the peak electric field location from the corner of the p-wellregion to the PS #1 region. In other words, the location of the peakelectric field in 4H-SiC during high drain bias operation has been movedfarther away from the gate oxide.

The advantage of this embodiment shown in FIG. A3 is that the breakdownlocation has been moved from the corner of the p-well region to the baseof the newly formed PS #1 region, which is both farther away from thegate oxide interface and deeper into the semiconductor. The electricfield in the gate oxide for the device structure incorporating the PS #1region is now significantly lower than the prior art device which isshown using device simulation using SILVACO ATLAS in FIG. A5 b.

An embodiment shown in FIG. A5 a is the cross-sectional unit cell of avertical power DMOSFET. The key regions of this device are a p-wellregion A503, which is formed by ion-implantation or epitaxial re-growthof a p-type species such as aluminum or boron. There is an N+ sourceregion A504, N− drift layer A502, and an N+ substrate A501. In the ONstate when a gate voltage is applied to the polysilicon gate A506, thecurrent flows vertically from the drain A501, through the inversionlayer which is formed at the top of the p-well layer A503, through theN+ source region A504, and out through the source metallization A508.There are several key features in a power MOSFET, which include thepitch of the unit cell, which is the repeat unit for the MOSFET, thechannel length, which is the portion of the p-well in which theinversion channel is formed, the distance between two successivep-wells, which is referred to as the JFET region or the JFET gap and thethickness of the gate oxide A505. Another feature is an ILD layer A507which is used to insulate the source interconnect metallization A508from the poly-silicon gate A506. Here a second P-type Sinker #2 (PS #2)region A510 under the N+ source region A504 is formed in addition to thePS #1 region A509 already described in an embodiment shown in FIG. A3.The PS #2 region is formed deeper than the p-well region A503 butshallower than the PS #1 region A509. For example, the PS #2 region A510can be made 20% deeper than the p-well region A503, while the PS #1region A509 can be made 20% deeper than the PS #2 region A510.

The presence of the PS #2 region results in a sharing of the peakelectric field under high drain bias operation between the PS #1 and PS#2 regions. The presence of the PS #2 region alleviates a local maximumof the peak electric field in the device structure under high drain biasconditions. In the prior art SiC MOSFET shown in FIG. A1 a, the designof the p-well region needs to satisfy conflicting requirements. Theelectrical dose in the p-well region under the N+ source region must behigh enough to prevent reach-through breakdown under off-state (highdrain bias) operation. But, at the same time, the surface doping of thep-well region must be low enough to maintain a reasonably low gatethreshold voltage and device ON resistance. In this context, the PS #2region introduced in this embodiment can be thought of as a secondaryp-well region and can be designed with the main purpose of preventingreach-through breakdown and for appropriately shaping the electric fieldprofile under high drain bias operation. The presence of the PS #2region frees up the constraint in the design of the primary p-wellregion, which can be used for ON-state related device metrics such asgate threshold voltage and ON resistance. Since with the introduction ofthe PS #2 region, p-well region does not have to be designed with theobjective of supporting the breakdown voltage it gives a lot moreflexibility to the designer to design the p-well region suitably forjust MOS channel formation.

Breakdown simulation FIG. A5 b of a SiC DMOSFET designed according tothe two embodiments described in FIG. A3 and FIG. A5 a shows that thepeak electric field location has been moved away from the corner of thep-well region to the center of the unit cell, which results in a lowerelectric field in the gate oxide layer. More specifically, using thestructural innovations disclosed in this document, one can achieve apeak electric field in the gate insulator of less than 3.5 MV/cm.

FIG. A6 a to FIG. A6 j describes the process of manufacturing thestructure shown in FIG. A5 a. The manufacturing process for a SiCDMOSFET is on a SiC substrate A601 and starts with using a 4H-SiCSi-face epi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1μm to 300 μm) for the epilayer A602 shown in FIG. A6 a. A blanket hardmask A603 comprising a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm is deposited in FIG. A6 b and thenpatterned using photolithography followed by a dry etch (using areactive ion-etch for example) as shown in FIG. A6 c. A P-typeimplantation A604 comprising Boron or Aluminum, at energies ranging from10 keV to 800 keV, at implant doses ranging from 10¹² cm⁻² to 10¹⁵ cm⁻²is performed in FIG. Abd to create a p-well A605 in FIG. Abe. The maskA603 is removed, and another hard mask layer A606 is depositedcomprising a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm and patterned in FIG. A6 f to define the N+source region. Notice that the center of the unit cell is masked forsource (SRC) implantation. The SRC (N+ source region) A607 is formed byimplantation of the n-type (N+) impurity A608 comprising nitrogen orphosphorus, at energies ranging from 10 keV to 500 keV, at implant dosesranging from 10¹³ cm⁻² to 10¹⁶ cm⁻² as shown in FIG. A6 g. The PS #2region A609 is formed by implantation of the P type impurity A610 asshown in FIG. A6 h. The deep PS #2 region can be formed usingion-implantation of aluminum or boron, which are commonly used p-typedopants in 4H-SiC. Boron can be advantageously used for forming thisdeep P-type Sinker region, since it has significantly higher implantrange as compared to aluminum, and deep implants using boron can beformed with lower ion-implantation energies. If using aluminum, the PS#2 region may comprise implant energies ranging from 100 keV to 1 MeVand implant dose ranging from 10¹³ cm⁻² to 10¹⁶ cm⁻². If using boron,the PS #2 region may comprise implant energies ranging from 50 keV to800 keV and implant dose ranging from 10¹³ cm⁻² to 10¹⁶ cm⁻². The samehard mask A606 utilized for forming the N+ source region is utilized fordefining the ion-implanted PS #2 region. In this case, the PS #2 regionis self-aligned under the N+ source region. The hard mask A606 isremoved and another hard mask layer A612 is deposited and patterned inFIG. A6 i. The PS #1 region A611 is formed by implantation of the P-typeimpurity A613 that may comprise aluminum or boron. Boron can beadvantageously used for forming the PS #1 region, since it hassignificantly higher implant range as compared to aluminum, and deepimplants using boron can be formed with lower ion-implantation energies.If using aluminum, the PS #1 region may comprise implant energiesranging from 100 keV to 1 MeV and implant dose ranging from 10¹³ cm⁻² to10¹⁶ cm⁻². If using boron, the PS #1 region may comprise implantenergies ranging from 50 keV to 800 keV and implant dose ranging from10¹³ cm⁻² to 10¹⁶ cm⁻². PS #2 region can be formed with a depth higherthan that of the p-well region but lower than that of the PS #1 region.As a second example of this embodiment, the PS #2 region can be made 20%deeper than the p-well region, while the PS #1 region can be made 20%deeper than the PS #2 region. In a related embodiment of this invention,the doping profile of the PS #1 and PS #2 regions can be formed withgradually decreasing doping concentration with a peak value at the SiCsurface. This graded doping profile for the PS #1 and PS #2 regions canprevent the formation of sharp corners in the 3-Dimensional landscapeand is beneficial for spreading the peak electric field during highdrain bias operation over a larger area which results in a higherbreakdown voltage. As examples, the PS #1 and PS #2 regions can beformed with a peak doping in the range of 10¹⁹ cm⁻³ to 10²⁰ cm⁻³ closeto the SiC surface and close to the N+ Source region, respectively. Thedoping concentration in the PS #1 and PS #2 regions may linearlydecrease as a function of depth into the silicon carbide to thebackground N-drift layer doping (in the range of 10¹⁴-10¹⁶ cm⁻³). Alinearly graded doping profile of the PS #1/PS #2 regions results in asufficient portion of the drain potential being supported within the PS#1/PS #2 regions and not only the N− drift region. This results in apseudo charge-balanced structure, which promotes breakdown over a largerSiC region, which results in a lower critical electric field atbreakdown and consequently a lower electric field in the gate insulator.The formation of the PS #1 region is followed by the removal of the hardmask A612 (dry or wet etch) in FIG. A6 j. The remaining steps in theprocess to create the structure shown in FIG. A5 a exactly follows thesame steps shown in FIG. A4 j to FIG. A4 r.

This “staircase” arrangement of the PS #1 and PS #2 regions with certaindepths can result in an even lower electric field in the gate oxideunder high drain bias operation. Yet another benefit of the PS #2 regionis that the p-well region can be made shallower and with a lower implantdose, which can reduce the device ON-resistance while simultaneouslyincreasing the gate threshold voltage, both of which are desirable froman applications standpoint. Thus, a better trade-off between theoff-state and on-state performance is enabled by the teachings of thisinvention.

An embodiment shown in FIG. A1 a is the cross-sectional unit cell of aSiC DMOSFET. The key regions of this device are a p-well region A703,which is formed by ion-implantation or epitaxial re-growth of a p-typespecies such as aluminum or boron. There is an N+ source region A704, N−drift layer A702, and an N+ substrate A701. In the ON state when a gatevoltage is applied to the polysilicon gate A706, the current flowsvertically from the drain A701, through the inversion layer which isformed at the top of the p-well layer A703, through the N+ source regionA704, and out through the source metallization A708. There are severalkey features in a power MOSFET, which include the pitch of the unitcell, which is the repeat unit for the MOSFET, the channel length, whichis the portion of the p-well in which the inversion channel is formed,the distance between two successive p-wells, which is referred to as theJFET region or the JFET gap and the thickness of the gate oxide A705.Another feature is an inter-layer dielectric (ILD) layer A707 which isused to insulate the source interconnect metallization A708 from thepoly-silicon gate A706. Here a trench A711 is etched into the N+ sourceregion A704 before implanting the PS #1 region A709. Here PS #2 regionA710 under the N+ source region A704 is formed in addition to the PS #1region A709. The PS #2 region is formed deeper than the p-well regionA703 but shallower than the PS #1 region A709. It is desirable to movethe electric field location as far away from the gate oxide interfacefor robust blocking capability of the device operation which improvesthe device reliability. Moving the breakdown location to the middle ofthe unit cell which is furthest away from the gate oxide A705 is anembodiment and by etching a recess trench A711 into the N+ source regionA704, you have effectively created a deeper PS #1 region than what wasin FIG. A3 and FIG. A5 a and moved the electric field even further awayfrom the gate oxide region.

The PS #1 region is designed with the greatest depth among the P-typeSinker regions. While boron implantation is one technique to create suchdeep P-type regions in SiC technology, when you consider the maximumrange of commercial ion-implanters available it becomes clear that ionimplantation in SiC is not an easy process and deep ion implantation isparticularly difficult. The function of the N+ source region which isalso referred to as source trench is that the PS #1 region can berealized with lower ion-implantation energies, which lowers the cost andtime of this implantation step, without any compromise in implantationdepth. In the embodiment herein, a trench can be dry etched into SiCafter the implantation of the N+ source region but before theimplantation of the PS #1 region. The same hard mask layer used for thePS #1 implant can be used for etching the source trench into the SiC. Inthis embodiment herein, the hard mask layer is first patterned, followedby dry etching the trench into SiC, which is immediately followed byion-implantation of the PS #1 region. The formation of the source trenchnaturally extends the depth of the PS #1 region. Hence, the PS #1 regioncan be realized with lower ion-implantation energies, which lowers thecost/time of this implantation step, without any compromise in implantdepth. The formation of the source trench also removes the N+ sourceregion and enables a direct connection of the P-well region with the N+source ohmic contact thereby grounding the p-well and shorting theP-well region with the N+ source region.

FIG. A1 b is the breakdown simulation of a SiC MOSFET designed accordingto Embodiment shown in FIG. A7 a which shows that the peak electricfield location has been moved away from the corner of the p-well regionto the center of the unit cell, which results in a lower electric fieldin the gate oxide layer.

FIG. A8 a to FIG. A8 bb describes the process of manufacturing the SiCDMOSFET structure shown in FIG. A7 a. The manufacturing process for aSiC DMOSFET is on a SiC substrate A801 and starts with using a 4H-SiCSi-face epi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1μm to 300 μm) for the epilayer A802 shown in FIG. A8 a. A blanket hardmask A803 comprising a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm is deposited in FIG. A8 b and thenpatterned using photolithography followed by a dry etch (using areactive ion-etch for example) as shown in FIG. A8 c. A P-typeimplantation A804 comprising boron or aluminum, at energies ranging from10 keV to A800 keV, at implant doses ranging from 10¹² cm⁻² to 10¹⁵ cm⁻²in FIG. A8 d is performed to create a p-well A805 in FIG. A8 e. The maskA803 is removed, and another hard mask layer A806 is depositedcomprising a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm and patterned in FIG. A8 f to define the N+source region. The N+ source region A807 is formed by implantation ofthe n-type (N+) impurity A808 comprising Nitrogen or Phosphorus, atenergies ranging from 10 keV to 500 keV, at implant doses ranging from10¹³ cm⁻² to 10¹⁶ cm⁻² as shown in FIG. A8 g. The PS #2 region A809 isformed by the implantation of the P-type impurity A810 in FIG. A8 h. Thedeep PS #2 region can be formed using ion-implantation of aluminum orboron, which are commonly used p-type dopants in 4H-SiC. Boron can beadvantageously used for forming this deep P-type Sinker region, since ithas significantly higher implant range as compared to aluminum, and deepimplants using boron can be formed with lower ion-implantation energies.The patterned hard mask A806 is removed from the top in FIG. A8 i.Another hard mask layer A811 is formed on the top in FIG. A8 j andpatterned in FIG. A8 k. The pattered mask A811 is used to etch A812 intothe substrate in FIG. A81 for creating a recessed region. The sourcetrench which is the recessed region A813 is formed in the substrate bypenetrating the source region in FIG. A8 m. A P-type implant A814 inFIG. A8 n is performed for creating the PS #1 region. If using aluminum,the PS #1 region may comprise implant energies ranging from 100 keV to 1MeV and implant dose ranging from 10¹³ cm⁻² to 10¹⁶ cm⁻². If usingboron, the PS #1 region may comprise implant energies ranging from 50keV to A800 keV and implant dose ranging from 10¹³ cm⁻² to 10¹⁶ cm⁻².The PS #1 A815 is formed by the self-aligned implantation of the p-typeimpurity in FIG. A80. As described more completely in the aboveembodiments, a linearly graded doping profile of the PS #1/PS #2 regionsmay be preferable utilized in lieu of a uniform (abrupt) doping profile,which results in a sufficient portion of the drain potential beingsupported within the PS #1/PS #2 regions and not only the N− driftregion. This results in a pseudo charge-balanced structure, whichpromotes breakdown over a larger SiC region, which results in a lowercritical electric field at breakdown and consequently a lower electricfield in the gate insulator.

PS #1 region is designed with the greatest depth among the P-type Sinkerregions using Boron implantation and the formation of the trenchnaturally extends the depth of the PS #1 region. The same hard mask usedfor etching the source trench A811 is utilized to define the PS #1region. As will be apparent to those in the field of this invention, theSiC trench can be formed using dry etching techniques including reactiveion-etching, inductively coupled plasma (ICP)-RIE, etc. While a 90°sidewall slope is shown for this trench region in FIG. A1 a, it isadvantageous to form the trench regions with lower angle (60-90°)sidewalls, since this will reduce the curvature of the PS #1 region, andtherefore improve the blocking performance of the device.

The patterned hard mask A811 is removed from the top in FIG. Abp. Anoxide layer A816 for gate oxide is grown in FIG. A8 q. A polysilicongate layer is deposited on the top A817 in FIG. A8 r. The polysiliconlayer may be deposited using PECVD or LPCVD. The polysilicon layer maybe degenerately doped using boron or phosphorus, either in-situ or in asubsequent step. In-situ doping may be performed by the addition of PH3precursor to the polysilicon deposition chemistry. Post-depositiondoping of polysilicon may be performed by depositing a layer of POCl₃followed by a drive-in step at temperatures ranging from 700-900° C. Ahard mask A818 is deposited on top and patterned as shown in FIG. A8 s.The polysilicon gate layer A817 is etched by using the patterned masklayer A818 in FIG. A8 t. The mask layer A818 is then removed from thetop in FIG. A8 u. An interlayer dielectric (ILD) layer A819 comprising50 nm-1000 nm thick silicon dioxide, silicon nitride, silicon oxynitridelayers or a stacked combination thereof is deposited on the wafer inFIG. A8 v. A hard mask A820 is deposited and patterned on top to definethe ILD opening in FIG. A8 w. The ILD layer is patterned using the hardmask A820 as shown in FIG. A8 x. Further the gate oxide A816 is etchedusing the same mask A820 in FIG. A8 y. The mask A820 is then removed inFIG. A8 z. A nickel silicide region A821 is formed on the exposed SiCsurface in FIG. A8 aa. Interconnect metal layers A822 (either Al or Agor Au) are deposited and patterned on the top and bottom of the chip in.FIG. A8 bb

The advantage of the embodiment herein is that by etching the sourcetrench region, prior to the implantation of the PS #1 region, thebreakdown location has been moved further into the semiconductor andfarther away from the gate oxide layer. Due to the inherent hardness ofsilicon carbide, the observed range (depth) of ion-implanted dopants issignificantly lower than in silicon. For example, forming deep (>0.3 μm)ion-implanted layers in silicon carbide requires extremely highimplantation energies (>300 keV), which requires doubly or triplyionized implantation species, which drastically reduces the throughputand increase the cost of these ion-implantation steps. Forming thesource trench as described in this embodiment obviates the need fordoubly/triply ionized implantation steps, and low-cost withhigh-throughput singly ionized implantation can be utilized forrealizing the PS #1 region.

FIG. A9 a and FIG. A9 b show the output and breakdown I-Vcharacteristics of 1200 V SiC DMOSFETs fabricated using the teachings ofthis invention. A specific on-resistance of 2.9 mΩ-cm2, gate thresholdvoltage of 3.0 V and breakdown voltages in the 1400 V-1500 V range areachieved.

FIG. A10 a and FIG. A10 b show the transfer (ID v/s VGS) characteristicsof 1200 V SiC MOSFETs fabricated using the teachings of this inventionare shown. The devices feature a gate threshold voltage of 2.9 V, asub-threshold slope of 150 mV/decade and a transconductance of 9.2 Smeasured at a drain current of 20 A.

FIG. A11 shows a single-pulse avalanche energy of 1.05 J is measured fora 1200 V SiC MOSFET fabricated using the teachings of this invention.This corresponds to an energy density of 15.4 J/cm², when normalized tothe total chip size (Avalanche Energy/Chip Size).

Embodiments relate to SiC DMOSFET power devices that can be used forfabricating self-aligned power MOSFETs.

An embodiment relates to increasing MOS channel density.

An embodiment relates to increasing effective field-effect mobility.

An embodiment relates to improved device reliability.

An embodiment relates to reducing ON resistance for a given chip size.

An embodiment relates to design and manufacturing of self-aligned powerdevices.

An embodiment relates to design and manufacturing MOS channels withsub-micron channel lengths.

An embodiment relates to removal of the parasitic N+ source regioninadvertently formed in the periphery of the device.

An embodiment relates to proper grounding of the p-well region with thesource ohmic contact in the active region of the device

An embodiment relates to removal of the parasitic NPN transistor formedin active and peripheral regions of self-aligned power devices.

An embodiment relates to a combination of an appropriately locatedsource trench and ion-implanted P+ plug region to enable propergrounding of the p-well region in the main active MOSFET region andremoval of a parasitic N+ source region formed in the periphery of theMOSFET

An embodiment relates to the formation of highly doped P+ plug regionsin the periphery of the MOSFET, especially under the gate pad and gatebus regions.

An embodiment relates to the suppression of false turn-on of the MOSFETduring fast switching transitions, at vulnerable locations, due tothreshold voltage lowering by the body bias effect.

An embodiment relates to the improvement of the maximum dV/dt rating ofthe MOSFET.

An embodiment relates to the improvement of the maximum avalanche energyrating of the MOSFET.

An embodiment relates to a dedicated process step being utilized formasking the implantation of the N+ source region in the deviceperiphery. The P+ plug region is formed first in this embodiment, whilethe source trench is created later on in the process at the ILD etchstep.

An embodiment relates to a dedicated hard mask applied after thesidewall spacer is formed and before the N+ source implantation. Thislayer prevents the formation of the N+ source region in the deviceperiphery and hence parasitic NPN structures in the device periphery areavoided.

An embodiment relates to a p-well region (P-Well #2) placed under the N+source region and electrically connected to the main p-well region. TheP-Well #2 provides additional electrical dose under the N+ sourceimplant, to prevent reach-through breakdown, especially after the sourcetrench formation.

An embodiment relates to a dedicated process step is utilized formasking the implantation of the N+ source region in the device peripheryas well as the N+ source region in the middle of the unit cell in theactive region to enable ohmic contact to the p-well region.

An embodiment relates to the formation of the segmented polysilicon gatemetallization in the periphery of the device instead of a contiguousgate bus which will decrease the gate-body capacitance of the MOSFET.

An embodiment relates to the decrease of the gate capacitance of theMOSFET.

An embodiment relates to the increase of the switching speed of theMOSFET.

The embodiments described herein show novel techniques for design andmanufacture of self-aligned SiC DMOSFET power devices. Due to limitedmobility achievable on SiC planar DMOSFET it is necessary to form n-wellchannels that are submicron length, so that the overall power and ONresistance of the MOSFET are not degraded.

In a MOSFET, the channel region is formed due to the offset between thep-well and the N+ source region. If the p-well and the N+ source regionsare formed by two separate masking steps, there can be lithographicmisalignment, which will result in asymmetric MOSFET channel lengths onthe two sides of a unit cell. In the embodiments described herein, tominimize this asymmetry in the MOS channel length, the p-well and the N+source implant are done in a self-aligned fashion. There are severaltechniques proposed in the literature for achieving self-aligned MOSFET,including ones that use an oxide spacer approach to form self-alignedp-well and N+ source regions, with the N+ source implant under p-wellimplant.

The embodiments herein describe several innovative techniques duringself-aligned channel formation that not only allow for the eliminationof the misalignment error during manufacturing of SiC MOSFETs, but alsoallow the possibility of reducing the channel length. The channellengths can be made smaller with narrow spacers. While the self-aligningprocess does provide the benefits of making an arbitrarily short channellength as well as eliminating misalignment, it does create some otherproblems with respect to the structure of the device outside the unitcells, because n+ regions are co-incident with the p-well regions.

In a conventional MOSFET where the p-well and the N+ source regions arenot self-aligned there is a dedicated masking step that will be used forforming the N+ source implant. In the embodiments described herein, in aself-aligned process, there is no dedicated masking step performing theN+ source implant. There is a dedicated masking step performing thep-well implant and followed by depositing the spacer, then etching thespacer followed by implantation for realizing the N+ source region.Everywhere there is a p-well region, there will be N+ source implant aswell. It is not desirable to have N+ source implant at the periphery ofthe device. It is important to terminate the device with just the p-typeimplant. The various embodiments described herein introduce techniquesin which one can get rid of the N+ source implants in the periphery ofthe device and replace it with some other regions.

The active region of the MOSFET is where the current conduction happenswhile the periphery region of the MOSFET is where the edge terminationof the device is provided to block any voltage. The gate pad and gatebus regions are also considered to be part of the peripheral regions ofthe MOSFET, for the purposes of describing the innovations in thisdocument. In an embodiment described herein, a combination of anappropriately located source trench and ion-implanted P+ plug regionenables proper grounding of the p-well region in the main active MOSFETregion and removal of a parasitic N+ source region formed in theperiphery of the MOSFET.

In a conventional MOSFET where there is no self-alignment, one can putthe source implant wherever needed and then create a region forgrounding the p-well. In a self-aligned DMOSFET since the source implantis self-aligned to the p-well, one would have not only the p-wellextending all the way to the periphery but also have the N+ sourceregion extend all the way to the periphery.

A parasitic NPN transistor exists in the active region of the MOSFETunit cell, formed by the N+ source region which acts like the N+emitter, the p-well region which forms the P-base and the N+ sourceohmic contact. In the periphery of the device and also under the gatepad metallization, there is no N+ source ohmic contact, and as a result,the emitter and base regions of the parasitic NPN transistor are notshorted in this region. The parasitic NPN transistor will not beactivated under normal DC or switching operation of the MOSFET, butoperating the device under extreme conditions can cause the triggeringof the parasitic NPN transistor.

In the active region, by creating the source trench one has shorted theemitter and the base of this parasitic NPN transistor and ensured thatin the active region of the device, this parasitic transistor will notturn ON. In the periphery of the device without a source trench, theparasitic NPN transistor exists even though it is shorted in the activeregion. It is possible that the parasitic NPN transistor may turn on inthe peripheral region and it is not desirable.

In the embodiment herein, by creating a source trench in the peripheryof the device and by providing a p-type implanted plug region, it isensured that the parasitic NPN transistor is completely removed. Sincethe emitter of this transistor has been completely removed there is nochance for any kind of parasitic BJT structure. Typically, a separatemasking step is carried out for masking the N+ source region from beingformed at these locations, but in the embodiment herein, the sourcetrench can be advantageously used for removing the parasitic N+ sourceregions and replacing them with a P+ plug region, which is self-alignedwith the source trench and electrically connected to the p-well region.

The doping concentration of the P+ plug or sinker region to be placed inthe device periphery can be advantageously be made very high. In doingso, another parasitic device effect described here can be avoided. In atypical power MOSFET device structure, moderately doped P-well regionsexist in the device periphery with or without a parasitic N+ Sourceregion, as described above. If the N+ Source region is co-incident withthe p-well region in the device periphery (i.e. the n+ source region isnot deliberately masked from the peripheral region), this results in apartial compensation of the p-type electrical dose of the p-well region.In either case, the p-well regions in the device periphery can beextremely resistive from an electrical standpoint. During switching ofthe power MOSFET from off- to on-state or vice-versa, the extremely highrate of change of drain voltage (or dV/dt) can result in capacitivecurrent flow through these resistive peripheral p-well regions, whichare only collected by the Source ohmic contact in the active region ofthe MOSFET. In other words, the capacitive current caused by the highdV/dt during device switching has to traverse an extremely long distancefrom the device periphery to the Source ohmic contact in the activeregion. The high resistivity of p-well regions in prior art MOSFETs canresult in a significant body bias effect, which has the result oflowering the device threshold voltage in those regions. So, theseportions of the device can exhibit a false turn-on, which can result inpremature device failure/destruction. The highly doped P+ plug regionsin this embodiment can alleviate the aforementioned effect by reducingthe amount of body bias developed in the peripheral regions of theMOSFET, which makes the MOSFET described in this embodiment moreresilient to dV/dt induced failure. In other words, the MOSFET structuredescribed in this embodiment will exhibit a higher dV/dt rating andavalanche energy density than prior art MOSFETs. More specifically, theMOSFET structures introduced in this embodiment will enable asingle-pulse avalanche energy density of over 15 Joules/cm2, whennormalized to the total chip area. This reduces the switching losses andincreases the circuit efficiency.

In an embodiment herein, a dedicated process step is utilized formasking the implantation of the N+ source region in the deviceperiphery. The P+ plug region is formed first in this embodiment, whilethe source trench is created later on in the process at the ILD etchstep.

In the embodiment herein, there is a source trench in the active regionof the device but there is no source trench in the periphery of thedevice. A dedicated masking step is interspersed between the formationof the sidewall spacer after the p-well implant and before the N+ sourceimplant to mask the N+ source implant from the peripheral regions of theMOSFET. The masking layer protects the periphery of the device from thesource implanted region. A second deeper p-well region (P-Well #2) isformed using the same masking step used for the N+ source implant. P+plug regions are formed in a later step in both the active regions ofthe device as well as the device periphery. A trench is later etchedthrough the N+ source regions at discrete locations in the active regionto contact the P+ plug regions, which get shorted to the N+ sourceregion by the ohmic or silicide metallization.

In an embodiment herein, the source region implantation is masked fromthe periphery of the device. A dedicated process step is utilized formasking the implantation of the N+ source region in the device peripheryas well as the N+ source region in the middle of the unit cell in theactive region to enable ohmic contact to the p-well region.

In an embodiment herein, the polysilicon metallization has beensegmented in the periphery of the device and is not one continuous layeras commonly seen. There is a dedicated masking step interspersed betweenformation of the sidewall spacer, after the p-well implant and beforethe implementation of the N+ source implant, to mask the N+ sourceregion, from both the peripheral regions of the device as well as theselected areas in the active region.

In an embodiment herein segmenting the polysilicon metallization reducesthe parasitic gate to body or gate to source capacitance which areparasitic capacitances. The islands of polysilicon are not disconnectedbut are connected at a position orthogonal to the plane of the drawing.Reducing the parasitic capacitance will enable the device to switchfaster, which will increase the circuit efficiency by reducing theswitching losses.

An embodiment shown in FIG. B2 is the unit cell and the device peripheryof a cross-sectional structure of a SiC DMOSFET. The key regions of thisdevice are a p-well region B203, which is formed by ion-implantation orepitaxial re-growth of a p-type species such as aluminum or boron. Thereis an N+ source region B204, N− drift layer B202, and an N+ substrateB201. In the ON state when a gate voltage is applied to the polysilicongate B208 the current flows vertically from the drain B201, through theinversion layer which is formed at the top of the p-well layer B203,through the N+ source region B204, and out through the sourcemetallization B211. In the OFF state or the blocking state, a voltage issupported across the p-well B203, N− drift layer B202 junction and thereis a PN junction which is formed between the p-well and the N− driftlayer. The voltage applied to the structure is supported across this PNjunction in the reverse bias. There are several key features in a powerMOSFET, which include the pitch of the unit cell, which is the repeatunit for the MOSFET, the channel length, which is the portion of thep-well in which the inversion channel is formed, the distance betweentwo successive p-wells, which is referred to as the junctionfield-effect transistor (JFET) region or the JFET gap and the thicknessof the gate oxide B207. Another feature is an ILD layer B209 which isused to insulate the source interconnect metallization B211 from thepolysilicon gate B208. A nickel silicide region B210 is formed on theexposed SiC surface.

Source trench regions B205 are realized by dry etching through the N+source layer at selected locations of the device, followed by a p-typeion-implantation step to realize P+ plug regions B206 under the sourcetrench. At the very center of the unit cell, there is a P+ plug layerB206 which is grounded with the N+ source metallization. The purpose ofthe P+ plug in SiC DMOSFET is to ground the p-well region with the N+source contact.

The formation of the source trench B205 device structure enables propergrounding of the p-well region in the main active MOSFET region andremoval of a parasitic N+ source region B204 formed in the periphery ofthe MOSFET. When the N+ source region is formed in a self-alignedfashion with the p-well region, the formation of the source trench afterthe N+ source region formation removes parasitic N+ source regions fromthe device periphery of the chip and from under the gate pad region.

In the embodiment herein, combination of the source trench and theion-implanted P+ plug region provides three important functions. Firstit provides proper grounding of the p-well region with the source ohmiccontact in the active region of the device and second it helps in theremoval of the parasitic N+ source region inadvertently formed in theperiphery of the device. Third, the highly doped P+ plug regionincreases the dV/dt rating of the MOSFET. In the embodiment herein bothof the above ensures the removal of the parasitic NPN transistor thatwould be otherwise formed in those regions.

While a parasitic NPN transistor also exists within the MOSFET unitcell, the N+ emitter (formed by the N+ source region) and the P-base(formed by the p-well region) are shorted by the N+ source ohmiccontact. However, in the periphery of the device and also under the gatepad metallization, there is no N+ source ohmic contact, and as a result,the Emitter and Base regions of the parasitic NPN transistor are notshorted in this region. While this parasitic NPN transistor will not beactivated under normal DC or switching operation of the MOSFET,operation of the device under extreme conditions like short-circuit oravalanche-mode results in the simultaneous presence of extremely highjunction temperatures, high dV/dt and high current densities, which cancause the triggering of the parasitic NPN transistor. Typically, aseparate masking step is carried out for masking the N+ source regionfrom being formed at these locations. But in the embodiment herein, thesource trench can be advantageously used for removing the parasitic N+source regions and replacing them with a P+ plug region, which isself-aligned with the source trench and electrically connected to thep-well region.

FIG. B3 a to FIG. B3 gg describes the process of manufacturing thestructure shown in FIG. B2. The manufacturing process for a SiC DMOSFETis on a SiC substrate B301 and starts with using a 4H-SiC Si-faceepi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1 μm to300 μm) for the epilayer B302 shown in FIG. B3 a. A blanket hard maskB303 comprising a chemical vapor deposition (CVD) deposited layer ofsilicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm isdeposited in FIG. B3 b and then patterned using photolithographyfollowed by a dry etch using reactive ion etching (ME) for example asshown in FIG. B3 c. Then p-well region is first formed byion-implantation or epitaxial growth using aluminum or boron as thep-type impurity. A p-type implantation B304 comprising boron oraluminum, at energies ranging from 10 keV to 800 keV, at implant dosesranging from 10¹² cm⁻² to 10¹⁵ cm⁻² in FIG. B3 d is performed to createa p-well B305 in FIG. B3 e.

A second hard mask layer B306 is deposited by a CVD deposited layer ofsilicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm in FIG.B3 f on top of the patterned first hard mask layer B303. This isfollowed by an anisotropic etching B307 in FIG. B3 g to form a sidewallspacer B306 as shown in FIG. B3 h.

A n-type implant B308 is done in FIG. B3 i for creating a N+ sourceregion B309 in FIG. B3 j. A N+ source region is formed byion-implantation or by epitaxial re-growth by a n-type impurity such asnitrogen or phosphorus. Thus, the source region B309 is formed in aself-aligned fashion with the p-well region B304. The first and secondhard mask layers B303 and B306 respectively, are removed in FIG. B3 k.While the aforementioned sequence of process steps constitutes onemethod to form self-aligned p-well and N+ source region, other methodsmay be employed to achieve the same result. For example, the N+ sourceregion B309 may first be formed after deposition and patterning of afirst hard mask layer, followed by further etch back of the first hardmask layer to then form the p-well region B304.

Another mask layer B310 is deposited by a CVD deposited layer of silicondioxide, silicon nitride, silicon oxynitride, or a metallic layer suchas nickel, with thickness ranging from 50 nm to 5 μm on the top in FIG.B31. The mask layer B310 is patterned in FIG. B3 m. The pattered maskB310 is used to etch into the substrate B311 in FIG. B3 n using dry etchmethod to create a recessed region within the SiC. The recessed regionis the source trench B312 formed in the substrate by penetrating thesource region in FIG. B3 o. The entire N+ source region is removed bydry etching at these selected locations of the device. Using the samehard mask B310, by implantation of a controlled dose of a p-typeimpurity B313 such as aluminum or boron in FIG. B3 p a P+ plug layer isformed in FIG. B3 q. The depth of the P+ plug layer B314 may preferablyexceed the depth of the N+ source implant and may even exceed the depthof the p-well region, in a particular implementation. A p-typeimplantation comprising boron or aluminum, at energies ranging from 10keV to 800 keV, at implant doses ranging from 10¹³ cm⁻² to 10¹⁷ cm⁻² inFIG. B3 p is performed to create a P+ plug region. The P+ plug regionB314 is realized under the source trench B312, which is electricallyconnected to the p-well region B304. The mask layers are B310 removed inFIG. B3 r. The wafer is annealed for activating the implantedimpurities.

The oxide layer B315 which is the gate oxide is formed by thermaloxidation or using CVD of a dielectric layer such as silicon dioxide,silicon nitride, silicon oxynitride, etc. in FIG. B3 s. The thickness ofthe gate oxide could range from 5 nm to 100 nm. Either dry or wetthermal oxidation could be used for oxide growth. Plasma-enhancedchemical vapor deposition (PECVD) or low-pressure chemical vapordeposition (LPCVD) could be used for gate oxide deposition. Apolysilicon gate layer B316 is then deposited in FIG. B3 t. Thepolysilicon layer may be deposited using PECVD or LPCVD. The polysiliconlayer may be degenerately doped using boron or phosphorus, eitherin-situ or in a subsequent step. In-situ doping may be performed by theaddition of PH3 precursor to the polysilicon deposition chemistry.Post-deposition doping of polysilicon may be performed by depositing alayer of POCl₃ followed by a drive-in step at temperatures ranging from600-900° C. A hard mask B317 is deposited by a CVD deposited layer ofsilicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm on thetop and patterned as shown in FIG. B3 u. The polysilicon layer B316 isetched by using the patterned mask layer B317 in FIG. B3 v. The masklayer B317 is then removed in FIG. B3 w. An ILD layer B318 comprising 50nm-1000 nm thick silicon dioxide, silicon nitride, silicon oxynitridelayers or a stacked combination thereof is deposited on the wafer inFIG. B3 x.

A hard mask B319 is deposited by a CVD deposited layer of silicondioxide, silicon nitride, silicon oxynitride, or a metallic layer suchas nickel, with thickness ranging from 50 nm to 5 μm on the top andpatterned on top to define the ILD opening in FIG. B3 y. The ILD layerB318 is etched using the hard mask B319 as shown in FIG. B3 z. Furtherthe gate oxide B315 is etched using the same mask B319 in FIG. B3 aa.The mask B319 is then removed in FIG. B3 bb. A nickel silicide regionB320 is formed on the exposed SiC surface in FIG. B3 cc. A mask layerB321 is formed by a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm on the top and patterned in FIG. B3dd. The ILD layer B318 is etched using mask B321 in FIG. B3 ee. The masklayer B321 is removed in FIG. B1 ff. Interconnect metal layers B322 ofeither aluminum or silver or gold is deposited and patterned on the topand bottom of the substrate in. FIG. B3 gg.

In the embodiment herein, the same final structure for forming thesource trench region and P+ plug region may be alternatively realizedusing a slightly different approach. As in the above method, the p-wellregion and the N+ source region are realized using a self-alignedprocess. However, in the embodiment herein, the P-plug layer may beformed first using deep boron or aluminum implantation as a buried layerthat is placed under the N+ source region. The dry etching for the N+source trench realization may be performed later on in the process. TheN+ source trench may be realized preferably after the high-temperatureannealing process in one example implementation.

An embodiment shown in FIG. B4 is the unit cell and the device peripheryof a cross-sectional structure of a SiC DMOSFET. The key regions of thisdevice are a p-well region B403, which is formed by ion-implantation orepitaxial re-growth of a p-type species such as aluminum or boron. Thereis an N+ source region B404, N− drift layer B402, and an N+ substrateB401. In the ON state when a gate voltage is applied to the polysilicongate B408 the current flows vertically from the drain B401, through theinversion layer which is formed at the top of the p-well layer B403,through the N+ source region B404, and out through the sourcemetallization B412. In the OFF state or the blocking state, a voltage issupported across the p-well B403, N− drift layer B402 junction and thereis a PN junction which is formed between the p-well and the N− driftlayer. The voltage applied to the structure is supported across this PNjunction in the reverse bias. There are several key features in a powerMOSFET, which include the pitch of the unit cell, which is the repeatunit for the MOSFET, the channel length, which is the portion of thep-well in which the inversion channel is formed, the distance betweentwo successive p-wells, which is referred to as the JFET region or theJFET gap and the thickness of the gate oxide B407. Another feature is anILD layer B409 which is used to insulate the source interconnectmetallization B412 from the polysilicon gate.

A dedicated masking step is interspersed between the formation of thesidewall spacer after the p-well implant and before the N+ sourceimplant to mask the N+ source implant from the peripheral regions of theMOSFET. A second deeper p-well region (P-Well #2) B405 is formed usingthe same masking step used for the N+ source implant. P+ plug regionsB406 are formed in a later step in both the active regions of the deviceas well as the device periphery. The P+ plug layer B406 is grounded withthe N+ source metallization. The purpose of the P+ plug in SiC DMOSFETis to ground the p-well region with the N+ source contact.

A trench B410 is later etched through the N+ source regions at discretelocations in the active region to contact the P+ plug regions, which getshorted to the N+ source region by the ohmic silicide B411metallization. The source trench B410 is etched into the SiC at selectedregions using the same hard mask used for patterning the interleveldielectric layer (ILD). In the embodiment herein, the source trench B410serves two functions. First the source trench reveals the surface of theP+ plug layer that was previously buried under the N+ source region forsubsequent contact with the ohmic silicide metal and second the sourcetrench reveals the sidewalls of the N+ source region for subsequentcontact with the ohmic silicide metal.

In the embodiment herein, a dedicated hard mask or photoresist layer isapplied after the sidewall spacer is formed and before the N+ sourceimplantation. This layer prevents the formation of the N+ source regionin the device periphery and parasitic NPN structures in the deviceperiphery are avoided

In the embodiment herein, a second p-well region (P-Well #2) B405 isplaced under the N+ source region B404 and electrically connected to themain p-well region provides additional electrical dose under the N+source implant, for preventing reach-through breakdown, especially afterthe source trench formation. The etching of the source trench mayinadvertently etch part of the primary p-well region under the N+ sourceregion, thereby locally reducing the p-well dose in this region,resulting in undesirable reach through breakdown at these locations.

FIG. B5 a to FIG. B5 gg describes the process of manufacturing thestructure shown in FIG. B4. The manufacturing process for a SiC DMOSFETis on a SiC substrate B501 and starts with using a 4H-SiC Si-faceepi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1 μm to300 μm) for the epilayer B502 shown in FIG. B5 a. A blanket hard maskB503 comprising a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm is deposited in FIG. B5 b and thenpatterned using photolithography followed by a dry etch using RIE forexample as shown in FIG. B5 c. Then p-well region is first formed byion-implantation or epitaxial growth using aluminum or boron as thep-type impurity. A p-type implantation B504 comprising boron oraluminum, at energies ranging from 10 keV to 800 keV, at implant dosesranging from 10¹² cm⁻² to 10¹⁵ cm⁻² in FIG. B5 d is performed to createa p-well B505 in FIG. B5 e.

A second hard mask layer B506 is deposited by a CVD deposited layer ofsilicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm in FIG.B5 f on top of the patterned first hard mask layer B503. This isfollowed by an anisotropic etching B507 in FIG. 5g to form a sidewallspacer B506 as shown in FIG. B5 h. A patterned mask layer B508 isdeposited by a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm on the top alongside the patterned mask layerwith the sidewall spacer in FIG. B5 i. The hard mask layer B508patterned over the peripheral regions of the device.

A n-type implant B509 is done in FIG. B5 j for creating a N+ sourceregion B510 in FIG. B5 k. A N+ source region is formed byion-implantation or by epitaxial re-growth by a n-type impurity such asnitrogen or phosphorus. The N+ source region B510 is formed in aself-aligned fashion with the p-well region B504. The source region isformed in a self-aligned fashion with the p-well region, while it ismasked from the peripheral regions of the device. A deep second p-wellregion (P-Well #2) B511 may be formed at the same time as the N+ sourceregion in FIG. B5 k. The buried P-well #2 region may be formed usingaluminum or boron, and placed under the N+ source region in the activearea of the device. The P-Well #2 region may be preferably formed usingthe p-type species boron which has a higher ion-implantation range ascompared to aluminum in SiC. It may not be necessary to mask the p-wellregion #2 from the peripheral regions of the device.

The first and second hard mask layers B503 and B506 respectively, areremoved in FIG. B51. Another mask layer B512 is deposited by a CVDdeposited layer of silicon dioxide, silicon nitride, silicon oxynitride,or a metallic layer such as nickel, with thickness ranging from 50 nm to5 μm on the top in FIG. B5 m. The mask layer B512 is patterned in FIG.B5 n. By implantation with a controlled dose of a p-type impurity B513such as aluminum or boron in FIG. B5 o a P+ plug region B514 buriedunder the N+ source region B510 is realized in FIG. B5 p. The P+ plugregion B514 is electrically connected to the p-well region B504 and theP-well #2 region B511. The P+ plug region may be formed deeper than thep-well and P-Well #2 regions. The mask layer B512 is then removed inFIG. B5 q and the implants are activated by high-temperature annealing.

The oxide layer B515 which is the gate oxide is formed by thermaloxidation or using CVD of a dielectric layer such as silicon dioxide,silicon nitride, silicon oxynitride, etc. in FIG. B5 r. The thickness ofthe gate oxide could range from 10 nm to 100 nm. Either dry or wetthermal oxidation could be used for oxide growth. PECVD or LPCVD couldbe used for gate oxide deposition. A polysilicon gate layer B516 is thendeposited in FIG. B5 s. The polysilicon layer may be deposited usingPECVD or LPCVD. The polysilicon layer may be degenerately doped usingboron or phosphorus, either in-situ or in a subsequent step. In-situdoping may be performed by the addition of PH3 precursor to thepolysilicon deposition chemistry. Post-deposition doping of polysiliconmay be performed by depositing a layer of POCl₃ followed by a drive-instep at temperatures ranging from 700-900° C. A hard mask B517 isdeposited by a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm on the top and patterned as shown in FIG. B5t. The polysilicon layer B516 is etched by using the patterned masklayer B517 in FIG. B5 u. The mask layer B517 is then removed in FIG. B5v. An ILD layer B518 comprising 50 nm-1000 nm thick silicon dioxide,silicon nitride, silicon oxynitride layers or a stacked combinationthereof is deposited on the wafer in FIG. B5 w. A hard mask B519 isdeposited by a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm on the top and patterned on top to define theILD opening in FIG. B5 x. The ILD layer B518 is etched using the hardmask as shown in FIG. B5 y. Further the gate oxide B515 is etched usingthe same mask B519 in FIG. B5 z. The hard mask that is used forpatterning the ILD layer B519 is used to realize the source trenchregion B520 by etching completely through the N+ source layer and intothe P+ plug layer in FIG. 5 aa. The mask B519 is then removed in FIG. B5bb. A nickel silicide region B521 is formed on the exposed SiC surfacein FIG. B5 cc. A mask layer B522 is formed by a CVD deposited layer ofsilicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm on thetop which is then patterned in FIG. B5 dd. The ILD layer B518 is etchedin FIG. B5 ee. The mask layer B522 is removed in FIG. B5 ff.Interconnect metal layers B523 of either aluminum or silver or gold isdeposited and patterned on the top and bottom of the substrate in. FIG.B5 gg. The ohmic contact to the N+ source regions is made through theetched sidewalls of the source trench regions in this embodiment, asopposed to the horizontal surfaces of the N+ source regions inconventional MOSFETs.

An embodiment shown in FIG. B6 is the unit cell and the device peripheryof a cross-sectional structure of a SiC DMOSFET. The key regions of thisdevice are a p-well region B603, which is formed by ion-implantation orepitaxial re-growth of a p-type species such as aluminum or boron. Thereis an N+ source region B604, N− drift layer B602, and an N+ substrateB601. In the ON state when a gate voltage is applied to the polysilicongate B607 the current flows vertically from the drain B601, through theinversion layer which is formed at the top of the p-well layer B603,through the N+ source region B604, and out through the sourcemetallization B610. In the OFF state or the blocking state, a voltage issupported across the p-well B603, N− drift layer B602 junction and thereis a PN junction which is formed between the p-well and the N− driftlayer. The voltage applied to the structure is supported across this PNjunction in the reverse bias. There are several key features in a powerMOSFET, which include the pitch of the unit cell, which is the repeatunit for the MOSFET, the channel length, which is the portion of thep-well in which the inversion channel is formed, the distance betweentwo successive p-wells, which is referred to as the JFET region or theJFET gap and the thickness of the gate oxide B606. Another feature is anILD layer B608 which is used to insulate the source interconnectmetallization B610 from the polysilicon gate B607. A nickel silicideregion B609 is formed on the exposed SiC surface. At the very center ofthe unit cell, there is a P+ plug layer B605 which is grounded with theN+ source metallization. The purpose of the P+ plug in SiC DMOSFET is toground the p-well region with the N+ source contact.

In an embodiment herein, a dedicated masking step is interspersedbetween the formation of the sidewall spacer after the p-well implantand before the N+ source implant to mask the N+ source implant from theperipheral regions of the MOSFET. This also masks the N+ source implantfrom selected regions in the active area of the device, which enablesohmic contact to the p-well or P+ plug region. This embodiment obviatesthe need for a source trench that is present in earlier embodiments.

In the embodiment herein, a dedicated hard mask or photoresist layer isapplied after the sidewall spacer is formed and before the N+ sourceimplantation which prevents the formation of the N+ source region in thedevice periphery and parasitic NPN structures in the device peripheryare avoided. The implantation of the N+ source region is avoided inselected areas in the active region and this enables ohmic contact tothe p-well or P+ plug regions without an intervening N+ source region.

FIG. B7 a to FIG. B7 ff describes the process of manufacturing thestructure shown in FIG. B6. The manufacturing process for a SiC DMOSFETis on a SiC substrate B701 and starts with using a 4H-SiC Si-faceepi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1 μm to300 μm) for the epilayer B702 shown in FIG. B7 a. A blanket hard maskB703 comprising a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm is deposited in FIG. B7 b and thenpatterned using photolithography followed by a dry etch using RIE forexample as shown in FIG. B7 c. Then p-well region is first formed byion-implantation or epitaxial growth using aluminum or boron as thep-type impurity. A p-type implantation B704 comprising boron oraluminum, at energies ranging from 10 keV to 800 keV, at implant dosesranging from 10¹² cm⁻² to 10¹⁵ cm⁻² in FIG. B7 d is performed to createa p-well B705 in FIG. B7 e.

A second hard mask layer B706 is deposited by a CVD deposited layer ofsilicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm in FIG.B7 f on top of the patterned first hard mask layer B703. This isfollowed by an anisotropic etching B707 in FIG. B7 g to form a sidewallspacer B706 as shown in FIG. B7 h. A hard mask layer B708 is depositedby a CVD deposited layer of silicon dioxide, silicon nitride, siliconoxynitride, or a metallic layer such as nickel, with thickness rangingfrom 50 nm to 5 μm and pattered in FIG. B7 i. The patterned mask layeris formed on the top alongside the patterned mask layer with thesidewall spacer B706. A n-type implant B709 is done in FIG. B7 j forcreating a N+ source region B710 in FIG. B7 k. A N+ source region isformed by ion-implantation or by epitaxial re-growth by a n-typeimpurity such as nitrogen or phosphorus. Thus, the source region B710 isformed in a self-aligned fashion with the p-well region B705 while it ismasked from the peripheral regions of the device, as well as selectedareas of the active region of the device, to enable contact with thesource ohmic metallization. The first and second hard mask layers B703and B708 respectively, are removed in FIG. B71. Another mask layer B711is deposited by a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm on the top in FIG. B7 m. The masklayer B711 is patterned in FIG. B7 n.

Using the hard mask B711, by implantation of a controlled dose of ap-type impurity B712 such as aluminum or boron in FIG. B7 o a P+ plugregion is realized in FIG. B7 p. The depth of the P+ plug layer B713 maypreferably exceed the depth of the N+ source implant and may even exceedthe depth of the p-well region, in a particular implementation. The masklayer B711 is removed in FIG. B7 q. The wafer is annealed for activatingthe implanted impurities.

The oxide layer B714 which is the gate oxide is formed by thermaloxidation or using CVD of a dielectric layer such as silicon dioxide,silicon nitride, silicon oxynitride, etc. in FIG. B7 r. The thickness ofthe gate oxide could range from 10 nm to 100 nm. Either dry or wetthermal oxidation could be used for oxide growth. PECVD or LPCVD couldbe used for gate oxide deposition. A polysilicon gate layer B715 is thendeposited in FIG. B7 s. The polysilicon layer may be deposited usingPECVD or LPCVD. The polysilicon layer may be degenerately doped usingboron or phosphorus, either in-situ or in a subsequent step. In-situdoping may be performed by the addition of PH3 precursor to thepolysilicon deposition chemistry. Post-deposition doping of polysiliconmay be performed by depositing a layer of POCl₃ followed by a drive-instep at temperatures ranging from 700-900° C. A hard mask B716 isdeposited by a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm on the top and patterned as shown in FIG. B7t. The polysilicon layer B715 is etched by using the patterned masklayer B716 in FIG. B7 u. The mask layer B716 is then removed in FIG. B7v. An ILD layer B717 comprising 50 nm-1000 nm thick silicon dioxide,silicon nitride, silicon oxynitride layers or a stacked combinationthereof is deposited on the wafer in FIG. B7 w.

A hard mask B718 is deposited by a CVD deposited layer of silicondioxide, silicon nitride, silicon oxynitride, or a metallic layer suchas nickel, with thickness ranging from 50 nm to 5 μm on the top andpatterned on top to define the ILD opening in FIG. B7 x. The ILD layerB717 is etched using the hard mask as shown in FIG. B7 y. Further thegate oxide B714 is etched using the same mask B718 in FIG. B7 z. Themask B718 is then removed in FIG. B7 aa. A nickel silicide region B719is formed on the exposed SiC surface in FIG. B7 bb. A mask layer B720 isformed by a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm on the top which is then patterned in FIG. B7cc. The ILD layer B717 is etched in FIG. B7 dd. The mask layer B720 isremoved in FIG. B7 ee. Interconnect metal layers B721 of either aluminumor silver or gold is deposited and patterned on the top and bottom ofthe substrate in FIG. B7 ff.

An embodiment shown in FIG. B8 is the unit cell and the device peripheryof a cross-sectional structure of a SiC DMOSFET. The key regions of thisdevice are a p-well region B803, which is formed by ion-implantation orepitaxial re-growth of a p-type species such as aluminum or boron. Thereis an N+ source region B804, N− drift layer B802, and an N+ substrateB801. In the ON state when a gate voltage is applied to the polysilicongate B807 the current flows vertically from the drain B801, through theinversion layer which is formed at the top of the p-well layer B803,through the N+ source region B804, and out through the sourcemetallization B810. In the OFF state or the blocking state, a voltage issupported across the p-well B803, N− drift layer B802 junction and thereis a PN junction which is formed between the p-well and the N− driftlayer. The voltage applied to the structure is supported across this PNjunction in the reverse bias. There are several key features in a powerMOSFET, which include the pitch of the unit cell, which is the repeatunit for the MOSFET, the channel length, which is the portion of thep-well in which the inversion channel is formed, the distance betweentwo successive p-wells, which is referred to as the JFET region or theJFET gap and the thickness of the gate oxide B806. Another feature is anILD layer B808 which is used to insulate the source interconnectmetallization B810 from the polysilicon gate B807. A nickel silicideregion B809 is formed on the exposed SiC surface. At the very center ofthe unit cell, there is a P+ plug layer B805 which is grounded with theN+ source metallization. The purpose of the P+ plug in SiC DMOSFET is toground the p-well region with the N+ source contact.

In an embodiment herein, a dedicated masking step is interspersedbetween the formation of the sidewall spacer after the p-well implantand before the N+ source implant to mask the N+ source implant from theperipheral regions of the MOSFET. This also masks the N+ source implantfrom selected regions in the active area of the device, which enablesohmic contact to the p-well or P+ plug region. This embodiment obviatesthe need for a source trench that is present in some earlierembodiments.

In the embodiment herein, a dedicated hard mask or photoresist layer isapplied after the sidewall spacer is formed and before the N+ sourceimplantation which prevents the formation of the N+ source region in thedevice periphery and parasitic NPN structures in the device peripheryare avoided. The implantation of the N+ source region is avoided inselected areas in the active region and this enables ohmic contact tothe p-well or P+ plug regions without an intervening N+ source region.

In the embodiment herein, the polysilicon metallization in theperipheral regions of the device B807 are segmented and not a contiguouslayer. In the embodiment herein, the formation of the segmented gatemetallization in the periphery of the device instead of a contiguousgate bus could significantly decrease the gate-body capacitance of theMOSFET. This could result in a significant increase of the switchingspeed of the MOSFET.

FIG. B9 a to FIG. B9 ff describes the process of manufacturing thestructure shown in FIG. B8. The manufacturing process for a SiC DMOSFETis on a SiC substrate B901 and starts with using a 4H-SiC Si-faceepi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1 μm to300 μm) for the epilayer B902 shown in FIG. B9 a. A blanket hard maskB903 comprising a CVD deposited layer of silicon dioxide, siliconnitride, silicon oxynitride, or a metallic layer such as nickel, withthickness ranging from 50 nm to 5 μm is deposited in FIG. B9 b and thenpatterned using photolithography followed by a dry etch using RIE forexample as shown in FIG. B9 c. Then p-well region is first formed byion-implantation or epitaxial growth using aluminum or boron as thep-type impurity. A p-type implantation B904 comprising boron oraluminum, at energies ranging from 10 keV to 800 keV, at implant dosesranging from 10¹² cm⁻² to 10¹⁵ cm⁻² in FIG. B9 d is performed to createa p-well B905 in FIG. B9 e.

A second hard mask layer B906 is deposited by a CVD deposited layer ofsilicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm in FIG.B9 f on top of the patterned first hard mask layer B903. This isfollowed by an anisotropic etching B907 in FIG. B9 g to form a sidewallspacer B906 as shown in FIG. B9 h. A hard mask layer B908 is depositedby a CVD deposited layer of silicon dioxide, silicon nitride, siliconoxynitride, or a metallic layer such as nickel, with thickness rangingfrom 50 nm to 5 μm and pattered in FIG. B9 i. The patterned mask layerB908 is formed on the top alongside the patterned mask layer with thesidewall spacer. A n-type implant B909 is done in FIG. B9 j for creatinga N+ source region B910 in FIG. B9 k. A N+ source region is formed byion-implantation or by epitaxial re-growth by a n-type impurity such asnitrogen or phosphorus. Thus, the source region B910 is formed in aself-aligned fashion with the p-well region B905 while it is masked fromthe peripheral regions of the device, as well as selected areas of theactive region of the device, to enable contact with the source ohmicmetallization. The first and second hard mask layers B903 and B908respectively, are removed in FIG. B91. Another mask layer B911 isdeposited by a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm on the top in FIG. B9 m. The mask layer B911is patterned in FIG. B9 n.

Using the hard mask B911 and by implantation of a controlled dose of ap-type impurity B912 like aluminum or boron in FIG. B9 o a P+ plugregion B913 is realized in FIG. B9 p. The mask layer B911 is removed inFIG. B9 q. The wafer is annealed for activating the implantedimpurities.

The oxide layer B914 which is the gate oxide is formed by thermaloxidation or using CVD of a dielectric layer such as silicon dioxide,silicon nitride, silicon oxynitride, etc. in FIG. B9 r. The thickness ofthe gate oxide could range from 10 nm to 100 nm. Either dry or wetthermal oxidation could be used for oxide growth. PECVD or LPCVD couldbe used for gate oxide deposition. A polysilicon gate layer B915 is thendeposited in FIG. B9 s. The polysilicon layer may be deposited usingPECVD or LPCVD. The polysilicon layer may be degenerately doped usingboron or phosphorus, either in-situ or in a subsequent step. In-situdoping may be performed by the addition of PH3 precursor to thepolysilicon deposition chemistry. Post-deposition doping of polysiliconmay be performed by depositing a layer of POCl₃ followed by a drive-instep at temperatures ranging from 700-900° C. A hard mask B916 isdeposited by a CVD deposited layer of silicon dioxide, silicon nitride,silicon oxynitride, or a metallic layer such as nickel, with thicknessranging from 50 nm to 5 μm on the top and patterned as shown in FIG. B9t. The polysilicon layer B915 is etched by using the patterned masklayer B916 in FIG. B9 u. In the embodiment described herein, the maskingstep used to pattern the polysilicon gate metal, results in a partiallysegmented polysilicon pattern. While not apparent in the cross-sectionalschematic, the disjointed gate fingers would be connected at a positionorthogonal to the plane of the drawing. The gate insulator B914 isetched in FIG. B9 v using the patterned mask B916. The mask layer B916is then removed in FIG. B9 w. An ILD layer B917 comprising 50 nm-1000 nmthick silicon dioxide, silicon nitride, silicon oxynitride layers or astacked combination thereof is deposited on the wafer in FIG. B9 x.

A hard mask B918 is deposited by a CVD deposited layer of silicondioxide, silicon nitride, silicon oxynitride, or a metallic layer suchas nickel, with thickness ranging from 50 nm to 5 μm on the top andpatterned on top to define the ILD opening in FIG. B9 y. The ILD layerB917 is etched using the hard mask as shown in FIG. B9 z. The mask B918is then removed in FIG. B9 aa. A nickel silicide region B919 is formedon the exposed SiC surface in FIG. B9 bb. A mask layer B920 is formed bya CVD deposited layer of silicon dioxide, silicon nitride, siliconoxynitride, or a metallic layer such as nickel, with thickness rangingfrom 50 nm to 5 μm on the top which is then patterned in FIG. B9 cc. TheILD layer B917 is etched in FIG. B9 dd. The mask layer B920 is removedin FIG. B9 ee. Interconnect metal layers B921 of either aluminum orsilver or gold is deposited and patterned on the top and bottom of thesubstrate in. FIG. B9 ff.

FIG. B10 is photograph showing a SiC DMOSFET fabricated using theteachings of these inventions and tested for single-pulse avalancheenergy test.

An embodiment described herein relates to design and manufacturing of ashort-channel SiC MOSFET.

An embodiment described herein relates to minimizing the DIBL effect ofthe high voltage, short-channel SiC MOSFETs.

An embodiment described herein relates to improved device reliability.

An embodiment described herein relates to reducing ON resistance for agiven chip size.

An embodiment described herein relates to design and manufacturing MOSchannels with sub-micron channel lengths.

The embodiments described herein achieve a device with both low enoughON resistance as well as, high enough short circuit withstand time.

The embodiments described herein relate to increasing the dopingconcentration of the channel locally in certain regions of the channel.The non-uniformly doped channels for SiC MOSFET provide better tradeoffbetween ON resistance, threshold voltage and short circuit withstandtime.

In the embodiments described herein relate to increasing the dopingconcentration in the channel locally such that they do not increase thethreshold voltage too much, but at the same time reduce the DIBL effectto achieve a better trade of.

In an embodiment described herein a p-type shielding layer called thep-shield is formed within the p-well region. The p-shield alwaysoriginates within the p-well region, but it can extend beyond thevertical extent of the p-well region.

In an embodiment described herein the bottom of the p-shield region canextended beneath the p-well.

In an embodiment described herein there can be multiple p-shieldregions. The doping concentrations in the different p-shield region canbe different from each other.

The embodiment described herein relates to device where a p-shieldregion is formed buried within the p-well structure. The p-shield regionalways originates within the p-well region, but can extend beyond thevertical extent of the p-well region.

In an embodiment herein, device structures can have multiple p-shieldregions. In these cases, their doping concentration profiles of thedifferent p-shield regions do not necessarily have be the same and canbe different from each other.

In the race to achieve a lower ON stage resistance in planar gate SiCMOSFET, especially with high breakdown voltage ratings, it's a commonpractice to make the channel lengths as short as possible because thisreduces a great part of the conduction loss that is associated with SiCchannels. The MOS mobility in SiC MOS structures is significantlysmaller as compared to those found in silicon MOSFETS and as a result toachieve a low enough ON resistance, one needs to make the channellengths quite short and sometimes in the sub-micron range.

As the channel lengths become shorter, short channel effects becomes aproblem in a SiC power MOSFET causing the drain induced barrier loweringeffect, also called the DIBL effect which is responsible for a lot ofreliability issues in SiC MOSFETs.

One associated problem is a roll off of the threshold voltage at highdrain bias where the designed device achieves a certain desiredthreshold voltage only at very low drain bias. But, as the drain biasapproaches it's blocking value, the threshold voltage gets reducedsubstantially which is undesirable since the channel could inadvertentlyturn on.

Also, a device suffering the DIBL effect has extremely large saturationcurrents under high drain bias. which results in excessive powerdissipation under short circuit load conditions. This results in a lowshort circuit withstand time. While limited MOS channel mobility of SiCMOSFETS can be overcome with short channel lengths, it is associatedwith problems due to DIBL effects.

One approach to mitigating this problem is to increase the dopingconcentration in the channel region which uniformly increases thethreshold voltage of the device. While this approach can reduce thesaturation drain current, this also increases the ON resistance of thedevice.

The embodiments described herein can help achieve a device with both lowenough ON resistance as well as, high enough short circuit withstandtime. While the conventional approach is to just uniformly increase thedoping concentration in the channel region, the embodiments describedherein do not increase the doping concentration of the other channeluniformly but increases it locally in certain regions of the channel.The non-uniformly doped channels for SiC MOSFET provide better tradeoffbetween ON resistance, threshold voltage and short circuit withstandtime.

In the embodiments herein the doping concentration are increased suchthat they do not increase the threshold voltage too much, but at thesame time reduce the DIBL effect and hence achieving a better trade of.

In an embodiment herein a p-type shielding layer called the p-shield isformed within the p-well region. The p-shield always originates withinthe p-well region, but in certain examples of this embodiment, it canextend beyond the vertical extent of the p-well region.

In an embodiment herein the bottom of the p-shield region can beextended further down and can reach beneath the p-well. In an embodimentherein there can be multiple p-shield regions. The doping concentrationsin the different p-shield region can be different from each other.

In an embodiment herein a p-type shielding layer called the p-shield isformed buried within the p-well region. The p-shield always originateswithin the p-well region, but in certain examples of this embodiment, itcan extend beyond the vertical extent of the p-well region.

In an embodiment herein the bottom of the p-shield region can beextended further down and can reach beneath the p-well. In an embodimentherein there can be multiple p-shield regions buried in the p-wellregion. The doping concentrations in the different p-shield region canbe different from each other.

A p-shield region is formed buried within the p-well structurere-enforces” the doping of the p-well region locally and provides bettershielding of the MOSFET channel at the surface, while minimizing theDIBL effect. In the embodiment described herein since the p-shield isnot directly connected to the channel, the p-shield does not change theVIE

An embodiment shown in FIG. C2 a is the half unit cell of across-sectional structure of a SiC DMOSFET. The key regions of thisdevice are a p-well region C203, which is formed by ion-implantation orepitaxial re-growth of a p-type species such as aluminum or boron. Thereis an N+ source region C205, N− drift layer C202, and an N+ substrateC201. In the ON state when a gate voltage is applied to the polysilicongate C207 the current flows vertically from the drain C201, through theinversion layer which is formed at the top of the p-well layer C203,through the N+ source region C205, and out through the sourcemetallization C210. In the OFF state or the blocking state, a voltage issupported across the p-well C203, N− drift layer C202 junction and thereis a PN junction which is formed between the p-well and the N− driftlayer. The voltage applied to the structure is supported across this PNjunction in the reverse bias. There are several key features in a powerMOSFET, which include the pitch of the unit cell, which is the repeatunit for the MOSFET, the channel length, which is the portion of thep-well in which the inversion channel is formed, the distance betweentwo successive p-wells, which is referred to as the junctionfield-effect transistor (JFET) region or the JFET gap and the thicknessof the gate oxide C206. Another feature is an ILD layer C208 which isused to insulate the source interconnect metallization C210 from thepolysilicon gate C207. Nickel silicide regions C209 are formed on theexposed SiC surface.

In the embodiments herein a p-type shielding layer which is calledp-shield C204 a is formed within the p-well region. The p-shield can belocated inside the p-well such that its lateral location of the pointwhose doping concentration is the highest as compared to the averagebackground doping concentration of the p-well is positioned within theboundary of the p-well. The p-shield region always originates within thep-well region. Point A and B are given as the reference points fordescribing how the doping profile of the implanted p-shield region lookslike.

The embodiment shown in FIG. C2 b is similar to that of FIG. C2 a exceptthat the bottom of the p-shield region C204 b is extended further downinto the p-well and can reach outside the p-well region.

In an embodiment shown in FIG. C2 c and FIG. C2 d the devices aresimilar to FIG. C2 a. The devices in FIG. C2 c and FIG. C2 d exemplifythe case where there are multiple p-shield regions namely C204 c andC204 d in FIG. C2 c and C204 e and C204 f in FIG. C2 d. In these cases,their doping concentration profiles of the different p-shield regions donot necessarily have be the same and can be different from each other.

In case of biasing the drain with high voltage, the p-shield that isformed in the middle of the channel can help in mitigating the expansionof the drain bias-induced depletion region, which eliminates the DIBLeffect. In addition to the mitigation of the DIBL effect, the p-shieldregion also provides a simple way for controlling the V_(TH) of theMOSFET, which enables improving the short circuit time (t_(SC)). Thep-shield enables a local increase of the doping concentration of thep-well at critical locations in the device structure, as opposed to anuniform increase of the p-well doping concentration. A better trade-offwith respect to lower Vth, ON resistance and better immunity toshort-channel effects is obtained by methods described in theembodiment. In the case, where the depth of the p-shield region isgreater than the p-well region, i.e. the p-shield extends beyond thep-well region in the vertical direction, the p-shield can also providebetter shielding of the electric field to the channel region whichfurther mitigates the DIBL effect in the channel. The device structurewith multiple p-shield regions can be designed with different dopingconcentrations in the different p-shield regions. In an embodimentdescribed herein, a higher doping concentration can be applied to thep-shield region/s close to the edge of the p-well (POINT A), while thep-shield regions closer to POINT B can be made with lower dopingconcentrations. This structure will have the benefit of a lower gatethreshold voltage as well as excellent immunity to short-channeleffects, for a given channel length.

FIG. C3 a to FIG. C3 u describes the process of manufacturing thestructure shown in FIG. C2 a. The manufacturing process for a SiCDMOSFET is on a SiC substrate C301 and starts with using a 4H-SiCSi-face epi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1μm to 300 μm) for the epilayer C302 shown in FIG. C3 a. A blanket hardmask C303 comprising a chemical vapor deposition (CVD) deposited layerof silicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm isdeposited in FIG. C3 b and then patterned using photolithographyfollowed by a dry etch using reactive ion etching (ME) for example asshown in FIG. C3 c. The p-well region is first formed byion-implantation or epitaxial growth using aluminum or boron as thep-type impurity. A p-type implantation C304 comprising boron oraluminum, at energies ranging from 10 keV to 1000 keV, at implant dosesranging from 10¹² Cm⁻² to 10¹⁵ Cm⁻² is performed to create a p-well C305in FIG. C3 d. The patterned mask layer C303 is removed in FIG. C3 e.

A hard mask layer C306 is deposited by a CVD deposited layer of silicondioxide, silicon nitride, silicon oxynitride, or a metallic layer suchas nickel, with thickness ranging from 50 nm to 5 μm in FIG. C3 f. Themask layer C306 is patterned using photolithography followed by a dryetch using ME for example in FIG. C3 g.

A p-type implant C307 is done as shown in FIG. C3 h for creating ap-type region inside the p-well called p-shield C308. The p-shieldregion C308 can be formed using aluminum or boron as the p-typeimpurity. The doping concentration in the p-shield region may be in therange of 1E16 cm-3 to 1E21 cm-3. The patterned mask layer C306 isremoved in FIG. C3 i.

A blanket hard mask C309 comprising a chemical vapor deposition (CVD)deposited layer of silicon dioxide, silicon nitride, silicon oxynitride,or a metallic layer such as nickel, with thickness ranging from 50 nm to5 μm is deposited in FIG. C3 j and then patterned using photolithographyfollowed by a dry etch using RIE for example as shown in FIG. C3 k.

A N+ source region C311 is formed by ion-implantation or by epitaxialre-growth by a n-type impurity such as nitrogen or phosphorus C310 inFIG. C31. The patterned mask layer C309 is removed in FIG. C3 m.

The oxide layer C312 which is the gate oxide is formed by thermaloxidation or using CVD of a dielectric layer such as silicon dioxide,silicon nitride, silicon oxynitride, etc. in FIG. C3 n. The thickness ofthe gate oxide could range from 10 nm to 100 nm. Either dry or wetthermal oxidation could be used for oxide growth. Plasma-enhancedchemical vapor deposition (PECVD) or low-pressure chemical vapordeposition (LPCVD) could be used for gate oxide deposition. Apolysilicon gate layer C313 is then deposited in FIG. C3 o. Thepolysilicon layer may be deposited using PECVD or LPCVD. The polysiliconlayer may be degenerately doped using boron or phosphorus, eitherin-situ or in a subsequent step. In-situ doping may be performed by theaddition of PH3 precursor to the polysilicon deposition chemistry.Post-deposition doping of polysilicon may be performed by depositing alayer of POCl₃ followed by a drive-in step at temperatures ranging from700-900° C. The polysilicon layer C313 is patterned in FIG. C3 p. An ILDlayer C314 comprising 50 nm-1000 nm thick silicon dioxide, siliconnitride, silicon oxynitride layers or a stacked combination thereof isdeposited on the wafer in FIG. C3 q. The ILD layer C314 is patterned inFIG. C3 r. The gate oxide C312 is patterned in FIG. C3 s.

Nickel silicide regions C315 are formed on the exposed SiC surface inFIG. C3 t. Interconnect metal layers C316 of either aluminum or silveror gold is deposited and patterned on the top and bottom of thesubstrate in. FIG. C3 u.

An embodiment shown in FIG. C4 a is the half unit cell of across-sectional structure of a SiC DMOSFET. The key regions of thisdevice are a p-well region C403, which is formed by ion-implantation orepitaxial re-growth of a p-type species such as aluminum or boron. Thereis an N+ source region C405, N− drift layer C402, and an N+ substrateC401. In the ON state when a gate voltage is applied to the polysilicongate C407 the current flows vertically from the drain C401, through theinversion layer which is formed at the top of the p-well layer C403,through the N+ source region C405, and out through the sourcemetallization C410. In the OFF state or the blocking state, a voltage issupported across the p-well C403, N− drift layer C402 junction and thereis a PN junction which is formed between the p-well and the N− driftlayer. The voltage applied to the structure is supported across this PNjunction in the reverse bias. There are several key features in a powerMOSFET, which include the pitch of the unit cell, which is the repeatunit for the MOSFET, the channel length, which is the portion of thep-well in which the inversion channel is formed, the distance betweentwo successive p-wells, which is referred to as the junctionfield-effect transistor region (JFET region) or the JFET gap and thethickness of the gate oxide C406. Another feature is an ILD layer C408which is used to insulate the source interconnect metallization C410from the polysilicon gate C407. Nickel silicide regions C209 are formedon the exposed SiC surface.

A p-shield region C404 a is formed buried within the p-well structure.In other words, the p-shield is formed beneath the SiC surface, wherethe MOSFET channel is located. The p-shield region always originateswithin the p-well region as seen in FIG. C4 a, but may in certainexamples of this embodiment the p-well region C404 b extend beyond thevertical extent of the p-well region as shown in FIG. C4 b.

The embodiment shown in FIG. C4 b is similar to that of FIG. C4 a exceptthat the bottom of the p-shield region C404 b is extended further downinto the p-well and can reach outside the p-well region.

Device structures shown in FIG. C4 c and FIG. C4 d are also similar tothe device in FIG. C4 a but they exemplify the case where there aremultiple p-shield regions. The devices in FIG. C4 c and FIG. C4 dexemplify the case where there are multiple p-shield regions namely C404c and C404 d in FIG. C4 c and C404 e and C404 f in FIG. 42d . In thesecases, their doping concentration profiles of the different p-shieldregions do not necessarily have be the same and can be different fromeach other. Point A and B in each of these figures are given as thereference points for describing how the doping profile of the implantedp-shield region looks like.

The p-shield in FIG. C4 a to FIG. C4 d “re-enforces” the doping of thep-well region locally and provides better shielding of the MOSFETchannel at the surface, while minimizing the DIBL effect. Since thep-shield is not directly connected to the channel, the p-shield in thiscase does not change the threshold voltage. The device shown in FIG. C4a provides the same kind of field shielding for mitigating the DIBL butdoes not require the change of the threshold voltage value where it isinevitable for the device of FIG. C2 a.

FIG. C5 a to FIG. C5 u describes the process of manufacturing thestructure shown in FIG. C4 a. The manufacturing process for a SiCDMOSFET is on a SiC substrate C501 and starts with using a 4H-SiCSi-face epi-wafer with suitable doping (10¹⁴-10¹⁸ cm⁻³) and thickness (1μm to 300 μm) for the epilayer C502 shown in FIG. C5 a. A blanket hardmask C503 comprising a chemical vapor deposition (CVD) deposited layerof silicon dioxide, silicon nitride, silicon oxynitride, or a metalliclayer such as nickel, with thickness ranging from 50 nm to 5 μm isdeposited in FIG. C5 b and then patterned using photolithographyfollowed by a dry etch using reactive ion etching (ME) for example asshown in FIG. C5 c. Then p-well region is first formed byion-implantation or epitaxial growth using aluminum or boron as thep-type impurity. A p-type implantation C504 comprising boron oraluminum, at energies ranging from 10 keV to 1000 keV, at implant dosesranging from 10¹² cm⁻² to 10¹⁵ cm⁻² is performed to create a p-well C505in FIG. C5 d. The patterned mask layer C503 is removed in FIG. C5 e.

A hard mask layer C506 is deposited by a CVD deposited layer of silicondioxide, silicon nitride, silicon oxynitride, or a metallic layer suchas nickel, with thickness ranging from 50 nm to 5 μm in FIG. C5 f. Themask layer C506 is patterned using photolithography followed by a dryetch using ME for example in FIG. C5 g.

A p-type implant C507 is done in FIG. C5 h for creating a p-type regioninside the p-well called p-shield C508. The p-shield region C508 can beformed using aluminum or boron as the p-type impurity. A p-shield regionis formed buried within the p-well structure. In other words, thep-shield is formed beneath the SiC surface, where the MOSFET channel islocated. The p-shield region always originates within the p-well region.The location of the p-shield region, which is controlled by carefuladjustment of the implant energies used for realizing the p-shieldregion.

The p-shield region may be created by a p-type ion-implantation stepcomprising boron or aluminum, at energies ranging from 25 keV to 800keV, at implant doses ranging from 10¹² cm⁻² to 10¹⁵ cm⁻². The patternedmask layer C506 is removed in FIG. C5 i.

A blanket hard mask C509 comprising a chemical vapor deposition (CVD)deposited layer of silicon dioxide, silicon nitride, silicon oxynitride,or a metallic layer such as nickel, with thickness ranging from 50 nm to5 μm is deposited in FIG. C5 j and then patterned using photolithographyfollowed by a dry etch using RIE for example as shown in FIG. C5 k.

A N+ source region C511 is formed by ion-implantation or by epitaxialre-growth by a n-type impurity such as nitrogen or phosphorus C510 inFIG. C5 l. The patterned mask layer C509 is removed in FIG. C5 m.

The oxide layer C512 which is the gate oxide is formed by thermaloxidation or using CVD of a dielectric layer such as silicon dioxide,silicon nitride, silicon oxynitride, etc. in FIG. C5 n. The thickness ofthe gate oxide could range from 10 nm to 100 nm. Either dry or wetthermal oxidation could be used for oxide growth. Plasma-enhancedchemical vapor deposition (PECVD) or low-pressure chemical vapordeposition (LPCVD) could be used for gate oxide deposition. Apolysilicon gate layer C513 is then deposited in FIG. C5 o. Thepolysilicon layer may be deposited using PECVD or LPCVD. The polysiliconlayer may be degenerately doped using boron or phosphorus, eitherin-situ or in a subsequent step. In-situ doping may be performed by theaddition of PH3 precursor to the polysilicon deposition chemistry.Post-deposition doping of polysilicon may be performed by depositing alayer of POCl₃ followed by a drive-in step at temperatures ranging from700-900° C. The polysilicon layer C513 is patterned in FIG. C5 p. An ILDlayer C514 comprising 50 nm-1000 nm thick silicon dioxide, siliconnitride, silicon oxynitride layers or a stacked combination thereof isdeposited on the wafer in FIG. C5 q. The ILD layer C514 is patterned inFIG. C5 r. The gate oxide C512 is patterned in FIG. C5 s.

Nickel silicide regions C515 are formed on the exposed SiC surface inFIG. C5 t. Interconnect metal layers C516 of either aluminum or silveror gold is deposited and patterned on the top and bottom of thesubstrate in. FIG. C5 u.

FIG. C6 a and FIG. C6 b are output characteristics of two 3.3 kV SiCMOSFETs fabricated using the teachings of this invention are shown. TheFIG. C6 a device features a gate threshold voltage of 2.5 V, while theFIG. C6 b device features a gate threshold voltage of 3.8 V. The highergate threshold voltage is achieved by employing the novel non-uniformlydoped channel concept introduced in this invention. A 156% increase ofgate threshold voltage is achieved for only a 11% increase ofon-resistance (Vgs=20 V), by using the device design concepts introducedin this invention.

FIG. C6 c is transfer characteristics of two 3.3 kV SiC MOSFETsfabricated using the teachings of these inventions.

FIG. C6 d is a short-circuit test measured for two 3.3 kV SiC MOSFETsfabricated using the teachings of this invention.

Other embodiments are also within the scope of the following claims.

Although, various embodiments which incorporate the teachings describedin detail herein, those skilled in the art can readily devise many othervaried embodiments that still incorporate these teachings. For example,a complementary SiC MOSFET device with a P+ substrate, P− drift layerand P+ source can be created in a N-well region. The embodimentsdescribed are all applicable to the complementary MOSFET as well.

In the embodiments and claims herein, the terms “first conductivity typeregion” and “second conductivity type region” are used to describen-type and p-type regions respectively for a N type device. For a P typedevice “first conductivity type region” and “second conductivity typeregion” are used to describe p-type and n-type regions respectively.

All documents (patents, patent publications or other publications)mentioned in the specification are incorporated herein in their entiretyby reference.

1. A device comprising a unit cell on a SiC substrate, the unit cellcomprising: a gate insulator film, a trench in a well region, and afirst sinker region of a second conduction type, wherein the firstsinker region has a depth that is equal to or greater than a depth ofthe well region; wherein the device has an on-resistance of less than 3milliohm centimeter squared, a gate threshold voltage of greater than2.8 Volt, a breakdown voltage of greater than 1450 Volt, and an electricfield of less than 3.5 megavolt per cm in the gate insulator film at adrain voltage of less than or equal to 1200 Volt.
 2. The device of claim1, wherein the device comprises a semiconductormetal-insulator-semiconductor transistor component.
 3. The device ofclaim 1, wherein the unit cell further comprises a semiconductor body ofa first conduction type that comprises a drift zone; the well region ofthe second conduction type next to an insulator-semiconductor interface;and a source region of the first conduction type formed within the wellregion.
 4. The device of claim 1, wherein a depth of the trench isgreater than or equal to a thickness of the source region.
 5. The deviceof claim 1, wherein the first sinker region is located below the trench.6. The device of claim 1, further comprising a second sinker region ofthe second conduction type.
 7. The device of claim 6, wherein a depth ofthe second sinker region is less than a depth of the first sinkerregion.
 8. The device of claim 6, wherein a depth of the second sinkerregion is greater than a depth of the well region.
 9. A devicecomprising a unit cell on a SiC substrate, the unit cell comprising: awell region, a source region, and a trench, wherein a depth of thetrench is greater than a depth of the source region; wherein the devicehas an avalanche energy of greater than 14 Joule per centimeter squared,calculated by dividing an avalanche energy in Joules by a total die areain centimeter squared.
 10. The device of claim 9, wherein the unit cellfurther comprises a first sinker region of a second conductivity type,wherein an avalanche failure is located within the unit cell.
 11. Adevice comprising silicon carbide, the device having a secondconductivity type shield region that is outside a junction field-effecttransistor region, wherein a doping concentration in a secondconductivity type well region within a MOSFET channel is non-uniform;wherein the device has a gate threshold voltage of greater than 3.8Volt, a breakdown voltage of greater than 4100 Volt at a gate to sourcevoltage of 0 Volt, an on-resistance of less than 12 milliohm centimetersquared, an electric field of less than 3.5 megavolt per centimeter atless than or equal to a drain voltage of 3500 Volt, and a short-circuitwithstand time of greater than 6 microsecond at a drain voltage of 1500Volt.
 12. The device of claim 11, wherein the second conductivity typeshield region is located inside the second conductivity type well regionin a manner that a lateral location of a point having a higher dopingconcentration than that of an average background doping concentration ofthe second conductivity type well region is positioned within the secondconductivity type well region.
 13. The device of claim 11, wherein thesecond conductivity type shield region extends beyond the secondconductivity type well region.
 14. The device of claim 13, wherein thesecond conductivity type shield region extends beyond a vertical extentof the second conductivity type well region.
 15. The device of claim 11,wherein the device comprises multiple second conductivity type shieldregions.
 16. The device of claim 11, wherein doping concentrationprofiles of the second conductivity type shield region in differentregions are different.
 17. The device of claim 11, wherein dopingconcentration profiles of the second conductivity type shield region indifferent regions are not substantially different.